You can see all the student's grades for all their classes using Synergy. (Note: This does depend on if the other teachers are synching with Synergy correctly.)
Log into Synergy
From the Home Screen, click on the class the student is in.
Click on the student's picture panel and pick grade book.
This will load a new screen. On this new screen, click All Classes
Now you can see all the student's grades. You can also click on the carrot to see a better breakdown of that student's grades in that class if that teacher is using categories.
If you want to switch students, there's a drop down menu on the right hand side of the screen.
To change classes, return to the home screen (top left corner) and follow the directions again.