I admit the FAQ does not fully set forth the rule. The rule is here:
https://ussoccer.box.com/s/wk8nm2mwq34yvbbu548pvwywmdkklzju. See, Article III, Section F, Paragraph 4 discussing the procedure for Academy players playing for HS teams. The answer is basically, the player must prove to US Soccer that playing HS Soccer is a fundamental part of their enrollment criteria at the private school.
4. Academy Players currently registered with an DA Club have committed to forgo participating in both the Academy and high school or middle school soccer.
a) Players who participate in high/middle school soccer are ineligible to participate in Academy practice and competition during that same season.
b) Only players who have been granted a waiver by the League Office will be allowed to remain on their clubs Academy roster during the high school soccer season (waivers are not available for middle school). This waiver must be received by DA office before September 1, 2018, to be considered for the roster exemption. The waiver eligibility guidelines are as follows:
(1) Players who receive consideration or financial aid to attend a private high school based on their participation with the soccer team are eligible
(2) Although clubs are not limited to a specific number of waivers, the number is naturally self-limiting to field a full team throughout the Academy season.
(3) Players registered with a Development DA Club will not be allowed to reenter the program during the same season after participating in the high school season with their current or a different DA Club.
c) The waiver must include the following points:
(1) Addressed to Academy Program from the Principal, Head Master or similar position
(2) Player name and high school
(3) States the player is expected to participate in high school soccer as part of their enrollment at this school and subsidized education
(4) States the term, semester and/or dates in which the player will be participating in high school soccer
(5) Signed and dated
(6) Sent to Academy Office for approval prior to the start of the high school season