Often, school districts use the student ID number on your student's ID card for Here Comes the Bus. Try that number when you are signing up, if the school district hasn't already provided you with a different ID number through email (when they emailed you initially about participating in the service). (See Where Do I Get My Here Comes the Bus School Code? for more information.)
If your student doesn't have an ID card or if your school district is using different ID numbers, check to see if you have received such an email from your school district, school, or transportation department — one that contains Here Comes the Bus information, including your student's ID number and your district's school code.
If you can't find the student ID in your emails from them, email or call your transportation department and request the ID to use for your student. (The contact information for your bus department should be on your school district's website.) They will probably send the information to the email address they have on file for your student rather than give you the number over the phone.