Of course it constitutes transphobia. The school is under no legal requirement not to use Soren’s chosen name. No state law, no regulation, no legally required district policy mandates that the school use names that appear on birth certificates. The school can use his chosen name merely by deciding to and there would be no negative repercussions.

The school is choosing to be transphobic on purpose, only because they want to. It wouldn’t cost them anything to be kind, and they have decided to be cruel instead.

As for Soren’s not updating his birth certificate yet, neither of us knows what his obstacles might be and neither of us is in a position to mandate what he does in that respect. It’s not up to the school either, which is perfectly legally free to call out his chosen name at graduation.