Opt-in educational programs of all sorts are usually heavily selection-biased toward students that are well-motivated and generally exceptionally easy to teach—good support from home, probably above-average SES, above-average IQ, et c.
Which doesn't mean such programs are useless, just that they're typically far less effective if you try to apply them to a representative slice of the overall student population.
Which doesn't mean such programs are useless, just that they're typically far less effective if you try to apply them to a representative slice of the overall student population.