Don't smoke in school. Period. If you smoke before you leave and it doesn't last long enough then you aren't smoking enough weed or at least good enough weed. When I smoked before school I could smoke a bowl at home and I'd be flying halfway through 2nd period.
If i smoke just before school i tend to have a big buzz for the first period and a pretty cool buzz for the second. personally i think smoking in school is stupid, for one, it runs out before half the day is through amd puts me in a daze, another point is its not worth losing education over... I would go maybe once a month blazed to school but not everyday. If i was to be blazed the whole day at school i'd need atleast 3 sessions, and i'd rather have none than one... its just the wrong time and the wrong place, leave it for after school.