Here's a layout of how to write a good email to a teacher: 

1. Your Email Subject
Give the Class Name & Period, and a couple words that refer to what the email will be about. 

example: Science P.2- Homework Question

2. Start respectfully
Dear Dr./Mr./Mrs/Ms. __________,

3. Writing the Actual Email

• Be direct and polite about letting them know what is going on and what you need.
  (I'm feeling confused about how to divide fractions, and wanted to ask if you can help me tomorrow.) 

• Try to keep slang and text shortening to a minimum to keep it 'professional'.
  (Can u help me? vs Can you help me?)

4. REREAD your email.
I know in your heart you don't think you made any typos, but let your brain double check it.

5. Sign your email!
With appreciation,/Sincerely,/Best,/Have a nice day, 

Your Name