First and foremost, well done for asking the question. There are probably many more ppl's out there who will have learned something from this,not to mention ATC who may revise their instruction when talking to club a/c.
It is a little more clear now.If you were given 1500'- to leave , then the 'no alt rest. must have referred to the FIR. ie no rest. other than remaining outside CAS laterally/vertically. Instructors can and do make mistakes. I can quote many an occasion when an instructor misinterprets a rule or restriction and unfortunately it is only when it gets messy and follow up action is taken that it comes to light. It sounds as if you have a good understanding of procedures. If you are concerned about any ATC instruction, you can always ask -or prefix c/s with 'student' which should alert ATC to the potential problem. You may occasionally hear exasperation in a controllers voice as we can be a moody lot but that is better than the alternative!!!