There generally aren’t problems as long as students don’t dodge times. My son was ahead, so in his 3rd year he couldn’t get a class because it was filled by graduating seniors. He took a graduate class instead and picked it up next term.
Honors at SLO does get priority registration, but his seniority put him ahead anyway. He had so many APs that choosing honors would have added 10 non-technical hours to his already 200 hour curriculum. So, he declined the invite. He did all honors math though which was also by invite, but without any other requirements.