The advanced and master classes are kind of misleading. A lot of units are just fine staying advanced because the master class either has no advantage in stat growths or wastes your time tutoring weapon types you'll never use. I'd also recommend limiting the number of students you poach from other houses to 2-3. You never have a map that requires more than 12 units on the field, and it kinda lessens the impact if you save everyone instead of facing them later on.
Also don't be like me and ignore the rusted weapons for 120+ hours. Often times they just repair into D weapons but they hid some cool shit in there as well.