I agree with alexseanchai re: variability. Is the school private or public, in US terms? Logistically, it'd be unusual for a public school district (funded by the state, plus sometimes small to medium local donations) to put a teacher-planning day on the first day of summer session; Monday would be pretty safe to assume for start of classes. On teacher planning days, the staff go in and students are off.

For private schools (separate tuition/fees, plus often substantial donations), anything goes.

Me: attended both, with a kid currently in the public system; most familiar with California metro regions.