”all” is singular or plural?

1) All (adjective) no counting - the noun after all must be a plural number. eg:
All the tables in the classroom must line up straight.
All students in school must wear uniform.
2) All (noun) - all = one (singular number) as you wrote: all is over.

"All" may take either singular or plural verbs depending on the meaning.
e.g.
All the money has been spent.
All of the students know the answer.