Huh, I've attended classes at two universities: one for my undergraduate and one while I was in HS, and neither had a system in place to explicitly bar you from registering for a class whose pre-reqs you didn't fulfill. At best you'd get a scary message confirmation message telling asking you "You don't meet the pre-reqs, you may be dropped from the class, are you sure?".

Also, at least when I was an undergrad, the Banner student system (from ellucian company) had no system in place for barring you from registering from classes. This was frequently the point of discussion between the professors that I TA'd for.