Yeah, this is pretty common. I used to work for a major K12 vendor (in over 1/3 of all schools in the US) and I heard about this practice all the time from different customers (e.g., schools and districts). Sometimes it's locked by grade level (concern about kids reading material with themes that are "too mature") and sometimes it's locked by reading level (poor understanding of how leveled reading works). It shouldn't be implemented this way, but that doesn't stop plenty of schools from doing exactly that.

These are so cute! I love how you've used negative space to add details, while still keeping a lovely simplicity to the shapes. This would be an awesome illustration style for a picture book.