you have to be enrolled to participate. The question becomes what the school’s requirements are. If the school is on semesters and allows mid-semester enrollment, that would allow a kid to participate. If they are on quarters, you could be enrolled after spring break.
How that fits with the timing of spring ball is anyone’s guess, but as a general rule the coaches don’t want a kid to show up one day and be running drills the next. They’ll want them to go through winter conditioning, assimilate into the team structure and gain some knowledge of the schemes they’ll be asked to run before they ever see any practice reps.