In California, high school sports are governed by CIF, of which there are several 'sections'. We found that the different sections have different rules on matters such as working with a high school coach in a travel or elite program, or competing in two sports during the same season or competing with a non-high school team ... in the same sport during the same season.
In our son's case, the CIF-Southern Section did not (and still does not) allow for participation (practice or play) in any
organized (amateur) outside (i.e., non-high school) sport/activity
during the high school season for that sport. Nor are they permitted to cross-over from another high school sport (e.g., basketball and that "kicking-the-round-ball" sport) until that high school team is completely out of competition (e.g, if the basketball team goes into CIF play-offs, a two-way player cannot participate in baseball
try-outs, practices or games in high school until the basketball team has completed their play-off run). I am pretty sure they can't even 'try out' for a non-high school team without jeopardizing their eligibility ... though I think they can attend a pro team pre-draft work-out. As far as I know, the only exception may be high school football where
I think spring football activities may be permitted during baseball season but I am not positive. (Of course that may have to do with the fact that so many high school baseball coaches are involved with football at their school ...
Other CIF sections may handle these matters differently as they are allowed. Can't speak to any but the Southern Section ...
)