I'm a student that does some teaching work as a kind of Saturday job. I'm part of a number of extra-curricular ensembles at school... it's not essential for a teacher to do this, but I think it is very important for broadening musical knowledge of the students. Certainly if the teacher didn't conduct the orchestra (for example) and left a sixth-former to do it, a) the ensemble wouldn't be as good and

most people would leave.
None of the peripatetic music teachers at my school help with the ensembles (it's just the Head of Music and the other academic teachers) but a few years ago, the singing teacher was in charge of the choir, and the choir was FANTASTIC. it's really gone downhill since then (although we've just got a new guy conducting it, and it's getting a lot better). it really helps to have a specialist helping with the ensembles (at the moment the orchestra conductor is a string player, and the wind and brass don't get as many useful tips as the string section do!)
On behalf of your school, I would like to urge you to get involved

Although it's entirely up to you of course.
PS please join...