Do whatever works best for you and your students in your classroom.
If standing 100% of the time is not working, let them sit some of the time.
If sitting 100% of the time is not working, let them stand some of the time.

Only you will know what works best for your situation. It's great to get opinions from others, but it's your program to teach. Even if you get a great clinician to come in to your classroom to help you, they will only see a snapshot of what is happening on a daily basis, so their advice may not necessarily be the best course of action.