You will need to perform a query, and depending how many entities returning all them in a single request will either not be possible or practical. You will then need to use cursors with the query.
You should read the section of Queries in the ndb docs - they are clear about what needs to be done - https://developers.google.com/appengine/docs/python/ndb/queries
A simple query for all items and to return the details you want as a list of Json records you would do the following, using the map method of a query, which calls the supplied function or classmethod. It doesn't expect a method of the entity thats why I don't use toJSON directly.
def callback(student):
return student.toJSON())
results = Student.query().map(callback)
You may need to fiddle with your toJSON method, have a look at what results looks like when you run it. results
may also need to explicitly converted to json, so you may want to defer the explicit json.encode till after after you have run the query.