As a native English speaker, I don't find this sentence weird, and as far as I can tell, it's completely correct.
It is a slightly less common use of the verb "are." As Tim Romano noted, the phrase "are to" can be replaced with "must" to make the sentence simpler to understand for non-native speakers, although "must" connotes a greater sense of urgency. "Should" would also be a valid replacement:
Those in wheelchairs should ring the bell.
Here's some other, similar examples:
Passengers with children are to board the plane first.
Anyone without valid identification is to (singular form) enter the second line and wait to be served.
As for your other question:
Does it mean that the bell is for the particular use of disabled people?
The sentence didn't specify that the bell is only for the use of the disabled people, just that people in wheelchairs should ring the bell.