When I arrived, there wasn’t any student in the classroom.
When I arrived, there weren’t any students in the classroom.
When I arrived, there wasn’t a student in the classroom.
In negative sentences and most questions, use any with plural countable nouns or uncountable nouns: any students, any suitcases and any money.
Use any with a singular noun when it means ‘it doesn’t matter which one’: ‘You can choose any song.’
See SOME