Is it possible to schedule "Do Not Disturb" on specific days?

I'm a student, and we're not allowed to have our phones in class at my school, so I like to schedule DND (Do Not Disturb) from when school starts to when it ends, in case I'm running late and don't have time to drop it off in my locker. That way I know it won't go off in my backpack. However, it gets annoying when I look at my phone Saturday or Sunday afternoon and realize I've missed a bunch of calls or texts because I forgot to turn off DND that morning. This problem would obviously be fixed if there was a way to set which days you want DND to activate in addition to the time, but I can't find the setting!

Is there a setting? If not, why isn't there? It doesn't sound like a hard feature to add; it's already been implemented into the alarm software. What repercussions could possibly come from adding such a handy feature?!