This is the 1511th most frequent Polish word.
trwają
"Trwają" means "they last" or "they are ongoing."
In this context, 'trwają' conveys that the work is actively being carried out currently.
Prace nad projektem trwają.
Work on the project is ongoing.
In this sentence, 'trwają' means 'are ongoing' and describes the continuous action of the lessons occurring in the school.
Lekcje trwają w szkole.
Lessons are ongoing at the school.
Here, 'trwają' highlights the duration and ongoing nature of the negotiations.