Spring Break is different for all the schools. Generally it is anytime from the beginning of March until the end of April, with the highest concentrations of them taking place around Easter, usually.
If Disney is charging peak season prices then that is Spring Break time.
The county I teach in doesn't get any days off in March. Our only spring break is 3 days off around Easter...which I think is in April this school year.
You should be fine on March 7th. There will moderate crowds but nothing overwhelming. Have a game plan for each park and use fastpasses and you'll be fine. Mid-March will start to get really busy.