Your friends ask you to miss an important class to see a movie. You would

The correct choice is: politely decline to go

An important class should not be missed for a movie, but there is no need to lecture friends or waver.

Saying no courteously keeps your priorities right and the friendship intact; ‘advise them not to go’ oversteps, and ‘think twice before deciding’ shows indecision about something clear.

Hence the best response is ‘politely decline to go’.