Learn Finnish: Opettaja oli tänään poissa. - The teacher was absent today.

oli is the past tense ("was") of the verb olla (to be) for a third-person subject like opettaja (the teacher). If the sentence were in the present tense ("The teacher is absent"), you would use on.

Does poissa mean "absent"? Why does it end in -ssa?

Yes, poissa means "absent" or "away". You might recognize the -ssa ending as meaning "in", like in talossa (in the house). poissa is related to the word pois (away), so it literally translates to something like "in a state of being away".

Could I put tänään at the end of the sentence?

Yes, Finnish word order is quite flexible. Opettaja oli poissa tänään is also a perfectly fine way to say it. Placing the time word tänään right after the verb is just a very common and natural word order in Finnish.

How would I change this sentence to say "The teachers were absent today"?

You would need to make both the subject and the verb plural. Opettaja becomes opettajat (the teachers), and the past tense verb oli (was) changes to its plural form olivat (were). The new sentence would be Opettajat olivat tänään poissa.