Learn Finnish: Koulussa on uusi opettaja. - There is a new teacher at the school.

Finnish doesn't use a placeholder word like English does in 'there is'. Instead, you just start with the location (koulussa), add the verb (on), and finish with the person or thing that is there (uusi opettaja).

Could I say Uusi opettaja on koulussa instead?

Yes, but it changes the meaning slightly. Starting with the person makes it a normal statement: 'The new teacher is at the school.' Starting with the location (koulussa) is how Finnish creates the 'There is...' meaning.

Why does koulu have the -ssa ending here?

The -ssa ending means 'in' or 'inside'. To make this type of 'there is' sentence, Finnish needs to establish the location right at the beginning. So koulu (school) becomes koulussa (in the school).

If there were multiple new teachers, would the verb change to ovat?

Actually, no! A special rule for these location sentences is that the verb is always the singular on (is), even if there are multiple things or people in that location.