I think the risk is that there is some systematic difference between those who chose to participate and the overall population of public Montessori kids. For instance, maybe those with high incomes disproportionately chose to participate, and Montessori strengthens learning for this group, but if we could measure the whole population the result is more mixed. It can't be a fully RCT if there's some kind of opt-in provision (which is not to say that an opt-in provision is bad, or a study that is not fully RCT is irrelevant).