OK - so that wasn't even too hard to find. The statutes are from the Californai education code (https://law.resource.org/pub/us/ccr/gov.ca.oal.title05.html):

California Ed Code:

§304. Leaving Room at Recess.
Every pupil shall leave the schoolroom at recess unless it would occasion an exposure of health.

§352. Detention During Recess or Noon Intermission.

A pupil shall not be required to remain in school during the intermission at noon, or during any recess.

Ironically, our highly regarded public school system has a policy directly in conflict with the law:

XXUSD code:

Recess Restriction
A teacher may restrict a student's recess time only when he/she believes that this action is the most effective way to bring about improved behavior. When recess restriction may involve the withholding of physical activity from a student, the teacher shall try other disciplinary
measures before imposing the restriction. Recess restriction shall be subject to the following conditions:
1. The student shall be given adequate time to use the restroom and get a drink or eat
lunch, as appropriate.
2. The student shall remain under a certificated employee’s supervision during the period of restriction.
3. Teachers shall inform the principal of any recess restrictions they impose.

I think I need to do some follow-up!

Sue