If you like doing the mission or you just find the subjects on the forum interesting, then it's worth it. If it's a pain for you to do, then it's not.
A teacher who starts complaining about you spending some time learning something, is a strange teacher. I think the same goes if it was a woodworking class you had decided to take in your spare time. Maybe if you used the words hacker or hack, he automatically thought 'No no no!'. It's words with many meanings and prejudices these days.
If you have trouble keeping up with your studies, I can understand if he wants you to spend some more time on your school things.
If you like doing it and it's not destructive for your current school studies, then it's not even a relevant question, go for it.
If you like computers and want to work with them in one way or the other, then it's always a good thing to have some basic programming skills. Or networking knowledge, or IT-Security knowledge, or what ever you find interesting that has something to do with IT... study printers, they are still a big mystery to me sometimes
(But they often are very badly secured