Yes everyone does do that. Most people will not take a tangential single sentence criticism of an institution to mean that you hold none of the typical accompanying political views and are just narrowly opining on your experience.
I would imagine a large majority of readers read your original post and immediately in their head thought, “are they one of those school voucher people” or something along those lines.
I don’t really understand why it’s necessary to assume anything at all. Can the statement not be taken at face value?
If we are all going around assuming 99% of the positions of people we are engaging with, what is the point of discussing anything?