Here's the Ohio High School rule on participation from their handbook. There's many more states but the reason I picked Ohio is because year in and year out they produce many high level youth soccer players. It doesn't seem to be a problem with them saying when it's High school season you need to be committed to your school. And yes Ohio has ECNL teams, two ODP programs(North and South).

"4) Non-Interscholastic Participation
4.1) A member of an interscholastic soccer squad (any student who has played in a scrimmage,
preview or regular season/tournament game as a substitute or starter) sponsored
by the Board of Education shall not participate in a non-interscholastic program
(tryouts, practice or contest) as an individual or a member of a team in the sport
of soccer during the school’s season (Sports Regulation 4.3).
4.2) A member of an interscholastic soccer squad (any student who has played in a scrimmage,
preview or regular season/tournament game as a substitute or starter) sponsored
by the Board of Education may participate in non-interscholastic soccer prior to
and after the school season under the following conditions:
4.21) The number of interscholastic soccer players from the same school on a noninterscholastic
squad is limited to five on the roster of that non-interscholastic
squad. A roster is defined as all the members of the entire squad on that noninterscholastic
squad.
Note: Graduating seniors who are participating on a non-interscholastic soccer
squad after their final interscholastic contest of the season in the sport of
soccer are exempt from this limitation.
4.22) There is no limit on the number of soccer squad members from the same
school participating on the same squad during the period June 1-July 31.
4.23) A player may continue to play with a non-interscholastic squad in a national
qualifying tournament in excess of the five player limitation after July 31
until the squad is eliminated but no later than Labor Day.
4.24) Non-interscholastic participation includes tryouts, training, practice and competition