First, you can use the Here Comes the Bus website or app only if your school district is participating in the service. If it is, your district's transportation department should have contacted all the students' parents and guardians with Here Comes the Bus information, including the school code. Check your emails from your school district, school, or transportation department for the code.
If you can't find the code in your emails from them, try your school district's web page, specifically in their Transportation Department section, and see if it has been posted there.
If you don't see your Here Comes the Bus school code on the district's website, email or call your transportation department and request the code. (The contact information for your bus department should be on your school district's website.)