A teacher uses his teaching time in activities and other things

Undoubtedly the basic job of a teacher is to teach the
classes allocated to him, but other things such as activities, supervising,
preparing lessons and so on should also not be neglected, because they have
a beneficial effect on the students and on making people like school. The
student may benefit from these activities more than the lesson itself. The
school administration should organize these things and distribute these
tasks in such a manner that it will not affect the teacher’s performance in
his class. 

If the teacher has been assigned these things by the
administration then there is nothing wrong with his doing them. 

But if he is doing them on his own initiative, then he is to
be appreciated for that, but he should strike a balance, and not cause the
students to miss out on their due of explanation and teaching that they
need, or ask for a reduction in the classes that he gives so that he will be
free for these activities. He has to combine the two interests. 

If the activities that he does are not important, or someone
else is giving them sufficient attention, or he is giving them too much
attention and wasting time with them, at the expense of doing his job in the
classroom, then undoubtedly this is neglecting his duty and betraying the
trust. “And Allaah knows him who means mischief from him who means good”
[al-Baqarah 2:220]. He should fear Allaah and get his priorities
straight. 

Explaining the importance of activities and the obligation of
cooperating in organizing them, Shaykh Ibn Jibreen (may Allaah preserve him)
said:  We should cooperate and help one another in important matters that
are needed in the school, such as preparing articles, researching issues,
taking part in school activities, writing announcements, messages and advice
to inspire the school and its members to advance, giving a strong impression
to visitors of the strength of its activities, advice and ideas. Undoubtedly
this cannot be achieved by one or two people, and the principal should not
neglect this aspect and sit alone in the office and limit himself to making
each teacher teach his subject in the classroom; rather he must do more than
that, and encourage more student activities, and motivate them to do
righteous deeds, and encourage the students, teachers and employees to
cooperate with one another, and make some effort and spend some time on
writing memos and the like which will attract attention and impress
visitors, and demonstrate their sincerity towards students and others. If
the principal of the school does not do that, then the teachers should offer
suggestions and show their willingness to cooperate with him in the
interests of all. If the principal asks them or some of them to do some
tasks, then they should hasten to fulfil that request. This is something
that is required of everyone, and it comes under the heading of cooperating
in righteousness and piety. If money is needed to sponsor these activities,
then it is the duty of the one who is able to hasten to do that. Similarly
if the principal asks one of them to buy something important in the
interests of the school, and he is one who knows how to buy it and it is not
a great hardship, then it is the duty of brotherhood and friendship to
respond to the request and not limit himself to giving lessons inside the
classroom. Although that is his basic duty, there are other activities that
may be connected to this activity. It is known that the teacher’s working
hours are like those of any other worker; he should be available for the
whole time, from beginning to end. So if he is employed full-time, he should
not disdain to work with others in the interests of all. End quote from
al-Ajwabah al-Fiqhiyyah ‘ala As’ilah al-Ta’leemiyyah wa’l-Tarwabiyyah,
p. 21 

And Allaah knows best.