I left out the following sentence.

Thank you.

1) I think you should show the text message to your teacher and ask his advice about what to do. (ask him for advice.... is it possible?)

All looks fine, yes.

Yes, it is possible to ask someone for advice about what to do in a situation. In this case, "ask his advice" means that you should seek guidance or suggestions from your teacher about how to proceed with the text message you left out. Asking for advice can be helpful when you are unsure of the best course of action and need some input or perspective from someone else.