ImExClass does not store any user data. All data is stored only in the spreadsheet, which is the property of the user who creates it, and in Classroom, which is also the property of the user.
All the permissions that ImExClass requires are exclusively to perform the functions that have been detailed (import and export grades from Classroom). In no case is the data taken from the data sheet or the GoogleClassroom. Thus, data security is provided by Google's servers.