Early in my ministry, I tried to form a youth orchestra that met year-round, or at least during the school year. But, between marching band season, and all the other demands on the students throughout the year, that plan was doomed to failure. The church I'm at now uses January-March to prepare the students for their spring break mission trips, which is another time of the year I don't dare add something else to their already busy schedules. I quickly realized that a year-round Youth Orchestra was a bad idea.
Please don't consider this as an excuse to not have a student group at all, but instead, consider having a project-oriented group. Our current Youth Orchestra begins rehearsals at the end of marching band season, and only has 4-5 rehearsals, before the group plays in Sunday morning worship in November, and for one of our many Christmas Eve Services. Rehearsals resume after spring break, when they rehearse four more times, then play in Sunday morning worship before school lets out for the summer.
This plan allows the students the invaluable experience of leading worship with their instruments, without extending them too thin, and it sets them up for success!