Hello, It depends on the State that you are licensed/certified/registered in, some States require PTA's to have CEU's some don't. If they do require you to have CEU's then you can usually attend the same classes that PT's go to.
There really is no black-and-white answer to your question. I would think there would be some "why is a PTA here" if it was a Con ed class on manual therapy (not massage) or evaluation techniques.
If it's ex intervention, general science/fondation information, then it shouldn't really be a problem.
Be sure to read the brochure for the class as many instructors will specifically state that PTA's/ATC's etc should not attend as the information/techniques given are not applicable to what their practice entails.