I use a mat stack for them to dismount onto after they can do 2 controlled back hip circles in a row. I make it just below back hip circle height from a solid front support. They should just clear the matstack with good form, so spot a few back hip circles at first to make sure they're good on form with the mat right there. After that, have them go for the dismount. They should land laying on their backs, with straight legs, feet together, toes pointed, arms straight overhead.
The mat being right there encourages them to push away from the bar aggressively from the correct position. Take away having to stick the landing and they can focus on the actual release. Spot the release at first too just to be sure they can get to the mat. After they can, I like to wait at the end of the mat and try to separate their legs from the ankle as soon as they land to check tightness.