They're first day in Kimetsu Academy (Ume is 16, Gyutaro is 18). They were in multiple high schools so this is the final one they move to. Their foster father (real one is unknown, mother died) Doma brought them to the school, you heard him talking to the principal

Doma: "Now I hope these two don't cause any trouble this time! Bye you two!"

Gyutaro: "Yeah, yeah whatever.. Bye dad.."

Ume: "Bye."

Doma leaves while the other two just walk in and notice you were eavesdropping