Constantly sleepy at school. Why does it start so early? Everyone knows teens stay up late.

Sleep issues. Any growing teen needs no less than 9 hrs sleep/d with extra needed for sports activity of heavy studying. If a kid is constantly sleepy they are trying to get by with less. Teens are preparing to assume responsibilities of adulthood. One of them is making needed modifications in their habits to meet schedules. You must show up for work on time or you will not have a job for long.
Get checked out. You can have sleep problems from snoring, apnea, restless legs, poor sleep hygeine, anxiety/depression, napping during the day, night seizures, noises in the neighborhood waking you up, bed bugs, waking up to use the bathroom, acid reflux, allergies, or any number of things. I recommend seeing a doctor to get checked out.