Earlier quoted context omitted.
1) We know it isn't because the same routine vaccination requirements have been around for decades. 2) They aren't forced. They are required to go to public school (with limited exemptions) but parents have other options, albeit more time-consuming and/or costly. The primary reason given is "pandemic-related interruptions in children’s medical visits as well as in-person schooling", and catching up with documentation…
Not all of the required "vaccines" have been around for decades. One of them is very new and that one seems to have a lot of controversy when it comes to kids and mounting questions on if it is safe or effective for said age group. Even mainstream news networks like the BBC are starting to air concerns from their own medical experts
The linked-to page says those are: "measles, mumps, and rubella (MMR) shot; the diphtheria, tetanus, and acellular pertussis (DTaP) shot; and the shots against poliovirus and varicella (chickenpox)."
Not COVID vaccination.