For teachers,how can I introduce a topic to opening a class?
It depends on the topic. Can you be more specific? WHat do you teach? Maybe you should establish a link with the previous lesson. If it is not possible, just say : Today we are going to study...
A sealed section inside. Doesn't have to be rude. Put on the cover 'such and such inside. Too controversial to print so it's sealed!'\n\nSomething like that anyway.