Graduate students (including international students on a F-1 visa) are able to work up to 100% in any combination of appointments (e.g, Associate, TA, Reader, GSR) on any given day in the summer term.
For example, a student (aka Academic Student Employee or ASE) could hold a 50% Associate appointment in combination with a 25% TAship and 25% GSR for a total of 100% employment in Session A. Then in Session B, the same ASE’s teaching assignment could be one 25% TAship which would allow his/her GSR appointment to be increased to 75%. Again, the total employment would not exceed 100% on any given day. No exception is required.