1)Hi,dear XXX
I'm not going to be in class tomorrow for some personal reason,so I'm wondering can you do me a favour to ask for leave?
Thank you so much.
XXX
2)Dear XXX,
Happy birthday!
I'm so looking forward to celebrate your birthday with you,it will be an amazing birthday party.But I've just caought a bad cold,I can't even move in my bed.So,I'm afraid I'm not gonna be there tonight,sorry about that.
Wish you have a great night.
Happy birthday again.
Regards,
XXX