they are newer alarms from after the 1994 ADA rule changes, after that all alarms had to have strobes in case of people with bad hearing. Your school just has an older system is why.
edit: in fact the old alarms (the school was built in from 1958) are still on the walls, but are disconnected.
That would explain why.
Different country. Don't really care for the deaf, lol.
Although, generally speaking, I'd imagine that even a deaf person would notice a fire alarm, when there are hoardes of people heading outside and lining up.
Unless they're like one person who manages to get themselves on their own with no one around.