While I don't think there is a way to 'disable this automatic parsing of the time' can you please specify how to recreate this issue, list the steps how you do it.
I try to re create it but I really don't understand how you get this issue. For what my understanding this is what you do:
.1 You have created a Class in Classroom called 1A. 2A atc.
.2 You enter ...
Woohoo! I just noticed today that Google Calendar now has the native functionality to display a 15-minute appointment at the proportional height you want it, that is, only taking up one-quarter of the hour-long slot. Finally! At least Google got there in the end, after years of feature requests. :)