Obviously there are no in-person groups in any context in Ontario right now, though The Rosebuds have managed to do some impressive remote work, which you should absolutely check out, but mostly, I’m limited to fantasizing about the day that playing with actual, real people is possible again, and hoping that when that day finally comes, I have enough interested students of similar levels that I can resume group classes.
Once students are advanced enough, a youth orchestra is one of the best options, in my opinion, but before that, in-studio group classes provide the groundwork for developing group playing ability. They are also fun and provide innumerable other benefits which include, but are not limited to, the following.
1) Group classes add a social aspect; practicing can be lonely. I had a student who was brought back from the brink of quitting by participating in a group class. It injected some fun and comradery into something that had become, at least for her, a bit pointless. If students aren’t performing and never play with their peers it can be easy for some to start to wonder what the point of practicing is.
2) Group classes help set goals; students see the importance of practice, because without it, they are unable to play through music. It’s one thing to routinely flub a passage in a lesson when the only witness is a teacher and maybe a parent. For most students, it is something else entirely to bomb in front of a roomful of their peers.
3) Rhythm and continuity; practicing alone means (at least in the absence of a metronome) students are able to rush through easy parts, slow down the hard parts, and in general disregard steady timing. Not that I condone this, but I guarantee it is happening. In a supervised group with a leader, their peers, and/or piano accompaniment, they are supported and held to the group tempo, and if not, the teacher can stop them to regroup. The students may drift apart at first, but with practice, and guidance, they gradually learn to stay together. It's much more fun than metronome work (though really they should be doing that too.)
5) This point is instrument specific. Violin is an orchestral instrument, and really needs a second (or more) instrument(s) to produce harmony (there are exceptions, but that is only possible at a fairly advanced level, and is a topic for another time). While piano can produce its own harmony, richer, more complex harmony can be produce with more instruments, and students never get the social aspect playing by themselves. Group playing is an important skill. Which brings us to the next point
6) Playing in a group is a very different skill than playing solo. There’s so much more going on, that usually, the first time students play with others they discover that even if they could get through their music on their own, suddenly it falls apart when put in a group context. When playing with others you have to listen to their part and pay attention to what’s going on around you in addition to the attention you must still give to your own part. If you don’t, you are almost guaranteeing that you will drift apart from the group and end up in a different part of the music from anyone else, and so, we come to point number 7.
7) Group playing teaches awareness of others and fosters a sense of community. In a group students have to constantly be aware of those around them. They have to listen, and work together. If they do not, the whole unit disintegrates. This is a great way to teach teamwork and awareness.
8) Memorization: (This is level specific as well as specific to certain groups, as advanced chamber music/orchestra music is not memorized). In most of my group classes in the lower levels, all music in should be memorized. Most of the students already have their music memorized, but this adds extra incentive in a more relaxed environment than performance. They play what they can, and drop out when they reach unfamiliar parts, but then they have a very clear idea of what pieces/parts need more home practice. Because it’s something that happens in a group, in general, they want to participate, so the impetus to memorize can come from them, and because they want to participate, rather than me just assigning memorization for what may seem to them to be arbitrary reasons. In the upper levels, chamber music is not usually memorized. The parts become complex, and by that point I’m usually confident in students’ memorization and listening skills and we are focusing on other aspects.