Salzburg is very nice, but it's a wee bit away from the Isar 😉
When it comes to museums, the "Deutsches Museum" is probably a must-see, myself I've been there twice already, first as a highschooler and then as a (partially) German-speaking adult. Hopefully the work to internazionalize the museum has progressed but 5 years ago knowing German was still a great help in enjoying the place!
If the weather is somewhat decent, you could spend a few hours walking along Englischer Garten, easily. If someone is feeling brave, you could also do some surfing right there, in the middle of the city! The famous Neuschwanstein is also in the area but you need a reservation and I'm not sure about the logistics because the school had rented a coach for the day.
Also, depending on the mood and the inclinations of the group, you could easily reach Dachau with the S-Bahn. I said "depending on the mood" because this place will definitely affect you but still it's one of those places you have to see for yourself!
Few jahre ago I went to München with my daughter and some other people. We went to Dachau. If you go with an appropiate guide, who explains you the inners of those "work camps", you will leave with a sunken heart.
But it's worth the sadness, because you learn a lot about "humans" and about yourself. If you can leave that place, knowing what happened there, and knowing that were "human beings" who were doing those things to other human beings (note the quoting) without any sadness in yourself, you have no empathy.
I feared she was too young for the experience, but it's something every youngster in the world should pass through (the visit, not the true experience).