This is an accesibility issue. In these cuarentine I have learned that the login experience is a pain point for people that never have use a email account or have low literacy skills either reading and writing and digital skills, especialy when the learner is in the inicial steps of reading and writing and the parents have never finish first grade. Like your flipgrid service these is a must. We can not bring support when the vast mayority of the people connect on the internet with a cellphone.
I am aware that the Azure active directory works with the email but in the edu context the email is irrelevant in comparison with the student id.