Recently application permissions were added to many education related Graph API request. However this is not yet the case for the Create educationCategory request.
Possible use case for accessing this request using Application Permissions
Assigning the same educationCategory to a Teams assignment created in different Teams of the same tenant would be useful to uniformly categorise assignments across the whole school, especially if those categories are used by a custom automatic processing process of the school.
Therefore, adding the same educationCategories to all relevant Teams using Application Permissions would be really useful, as otherwise every teacher has to grant this permission separately.
Another alternative would be that Teams supports some global educationCategories that are availably by default for all Teams.