I would like to say: "i´m great, thanks mom, how about you? school is fine, but i don´t think that i can finnish it this year. how is the weather? say hi to B. kisses."

So this is my attempt:"Cok iyiyim, tesekkür anne, ya sen? Okul iyi, ama bu sene bitecek sanmiyorum. Havalar nasil? B selam söyle. Öptüm."

So is this correct? (I don´t know if sanmak is the right verb here)

And can i say "söyle" when i´m writing to my mother-in-law or should i be more polite?

Thanks in advance!

Very good

If you want to use the polite form:

"Cok iyiyim, tesekkürler anne, ya siz? Okul iyi, ama bu sene biteceğini sanmiyorum. Havalar nasil? B´ye selam söyleyin. Öptüm."

(I don´t know if it is necessary though, although respect is nice, using the "siz" form also creates a little bit of distance)

Thank you so much, you have been very helpful