As per as my concern, I am sending to you- this sentence is sufficient because your email Id is already representing you. But generally, we use this word (sending) when we attach a file with mail. if you are writing only a message to your friends/ relative then we needn't write this sentence. Now you can examine yourself, why you are writing this sentence.

both sentences have own meaning, the first sentence- How many students are there, in Paul's class? it shows that Mr. paul teaches the student in his class. and the second sentence how many students are in paul's classes? it shows the present of student in his class.

I looked for my bag - this sentence belongs to simple past. whenever we finish our work in the past at the specific time. I was looking for its - this sentence belongs to past continuous. We use was/ were when we do our work in continuously in the past. we make our sentence with was/were+ ing in the main verb.

No, both are the different thing. According to your above sentence -Be seeing you, you are using present continuous tense- it means you doing the work in continuously. but when you are saying see you soon- it mean you will meet/see after a day or after a certain time. it mean you see him in future but the time is not fixed.

My reading yesterday

Yesterday I took a book named " La increíble y triste Historia de la cándida Eréndira y su abuela desalmada" written by Gabriel García Márquez. He was a famous Colombian writer. I read the first story of the book, named " Un señor muy viejo con unas alas enormes" ( A very old man with huge wings) and it about the man with wings who arrives the garden of the House of Family.The family was terrified, but they can not communicate with him because the man speaks the different language. However, the man was not harmful. The family contact with a neighbor and told them, the man was an angel...This story will continue...
I only want you to correct me..thanks