What does lofs 21 or over school bus only?

"LOFS 21 or over School Bus Only" likely refers to a local ordinance, freeway, or street where only school buses and vehicles with 21 or more passenger seats are permitted. The "LOFS" part isn't a standard abbreviation, so it's specific to that location's signage. It's restricting access to primarily large vehicles used for transporting people, likely to improve safety for students and reduce congestion.