The Prodigy Math Game is well-known amongst educators for its engaging math-based games. Further, its privacy policy allows for teachers to enroll students with just first and last name.
If you choose to use it, enter your students’ first names, then give them all a generic last name. You could go with something like “Smith”, or you could use the name of your school “Blakely” or “Sutherland”. This protects their privacy as no other information will be collected with respect to the students.