My son had a bar mitzvah a few weeks ago where instead of a bo bayom the class was invited to Shabbos lunch. I assume cost wise it evened out to making 2 separate events. If you don’t want to do that but do want a class/friends party you can do a melave Malka.
We had this. My son wanted a weekday seuda with his friends like everyone else in his class so we randomly picked a date the following week. On Shabbos we had a large kiddish for our friends and family seudas.
