The teacher asks them to send blessings upon the Prophet 300 times before each lesson

Always sending this number of blessings upon the Prophet (peace and blessings of Allaah be upon him) before the lesson is not the way of the Prophet (peace and blessings of Allaah be upon him) or of his companions or those who followed them in goodness. Whatever is like that is an innovation (bid’ah) which the Prophet (peace and blessings of Allaah be upon him) forbade and warned us against when he said: “Beware of newly-invented matters, for every newly-invented matter is an innovation and every innovation is a going astray.” Narrated by al-Tirmidhi (2600), Abu Dawood (3991), and Ibn Maajah (42); classed as saheeh by al-Albaani in Saheeh al-Jaami’ (2549). 

And the Prophet (peace and blessings of Allaah be upon him) said: “Whoever does a deed that is not in accordance with this matter of ours will have it rejected.” Narrated by Muslim (1817). 

The reason why this action is an innovation is that worship must be something that is prescribed in its nature, the way it is done, the time when it is done, and the number of times it is done. Allaah can only be worshipped in the ways that He has prescribed in His Book or on the lips of the Messenger of Allaah (peace and blessings of Allaah be upon him). 

Dhikr may be prescribed in principle, but people start to specify the way in it is to be done, or the place or time or number, that may bring it under the category of innovation. 

This is indicated by the report narrated by al-Tirmidhi (204) from ‘Amr ibn Salamah who said: We were sitting at the door of ‘Abd-Allaah ibn Mas’ood before Fajr prayer, when he came out. We walked with him to the mosque, then Abu Moosa al-Ash’ari came to us and said: Has Abu ‘Abd al-Rahmaan come out to you yet? We said: No. So he sat with us until he came out, and when he came out we all stood up. Abu Moosa said to him: O Abu ‘Abd al-Rahmaan, just now I saw something in the mosque that I have never seen before, but I do not think it was anything but good. He said: What was it? He said: If you live, you will see it. He said: In the mosque I saw some people sitting in circles waiting for the prayer. In every circle there was a man, and in their hands they had pebbles. He would say: Say Allaahu akbar (Allaah is Most Great) one hundred times, and they would say Allaahu akbar one hundred times. He would say: Say Laa ilaaha ill-Allaah (there is no god but Allaah) one hundred times, and they would say Laa ilaaha ill-Allaah one hundred times. He would say:  Say Subhaan-Allaah (Glory be to Allaah) one hundred times, and they would say Subhaan-Allaah one hundred times. He said: What did you say to them? He said: I did not say anything to them; I was waiting to see what you think and I waited for your command. He said: Why did you not tell them to count their bad deeds and guarantee to them that their good deeds would not be wasted? Then he moved on and we moved on with him until he came to one of those circles, and he stood over them and said: What is this that I see you doing? They said: O Abu ‘Abd al-Rahmaan, these are stones with which we count the takbeers (‘Allaahu akbar’), tahleel (‘Laa ilaaha illa-Allaah’) and tasbeeh (‘Subhaan-Allaah’). He said: Count your bad deeds, for I guarantee to you that none of your good deeds will be lost. Woe to you, O ummah of Muhammad! How quickly you have become doomed! His companions are still alive and his cloak has not worn out, and his vessel is not yet broken. By the One in Whose hand is my soul, you are either following a way that is more guided that the way of Muhammad or you are opening the door to misguidance. They said: By Allaah, O Abu ‘Abd al-Rahmaan, we intended nothing but good. He said: How many of those who intended good did not achieve it. The Messenger of Allaah (peace and blessings of Allaah be upon him) told us that some people would recite the Qur’aan and it would not go any further than their collarbones. By Allaah, I do not know, perhaps most of them are from among you. Then he turned away from them and ‘Amr ibn Salamah said: I saw most of those circles on the side of the Khawaarij fighting us on the day of al-Nahrawaan. 

Think about this attitude of Abu Moosa and ‘Abd-Allaah ibn Mas’ood, and how they denounced this method which was not done by the Prophet (peace and blessings of Allaah be upon him) or his companions, although dhikr is basically prescribed, praiseworthy and encouraged. 

The scholars pointed out that doing an act of worship at a specific time or in a specific place, or in a specific manner, that has not been narrated, is regarded as innovation. In that case it is called bid’ah idaafiyyah (an innovation by adding to something that is basically prescribed). It is prescribed in principle but is rejected because of the way in which it is done. 

Al-Shaatibi (may Allaah have mercy on him) said: Bid’ah (innovation) refers to something that is newly invented in matters of religion that appears similar to that which prescribed, by which people intend to go to extremes in worshipping Allaah. 

This includes adhering to certain ways and manners of worship, such as reciting dhikr in unison, or taking the birthday of the Prophet (peace and blessings of Allaah be upon him) as an Eid, and so on. 

It also includes doing certain acts of worship at certain times for which there is no basis in sharee’ah, such as always fasting on the fifteenth of Sha’baan (yawm al-nusf min Sha’baan) and spending that night in prayer. 

End quote from al-I’tisaam (1/37-39). 

Sending blessings upon the Prophet (peace and blessings of Allaah be upon him) is one of the best acts of worship and one of the greatest means of drawing closer to Allaah, but always doing that before every Qur’aan class, with this specific number, is something that is not narrated (from the Prophet (peace and blessings of Allaah be upon him)), so it is an innovation, even if the one who does it intends well. How many of those who intended good did not achieve it, as Ibn Mas’ood (may Allaah be pleased with him) said. 

What you must do is advise this teacher and explain that what he is doing is not Sunnah, rather it is bid’ah. If he responds, then praise be to Allaah, but if he does not respond and you can learn how to recite Qur’aan from someone else, then you should shun him as a rebuke to him and so as to prevent innovation entering schools at his hands.   

May Allaah help us and you to love the Sunnah and defend it, and to love the Prophet (peace and blessings of Allaah be upon him) and his companions. 

See also questions no. 20005, 21902 and 22457.
And Allaah knows best.