starlight you're spot on.
I have 8 years experience as a TA and a SEN TA each day. My day was split between children and classes.
Very oversimplified; my schools expectation was that a TA is there to support the teacher. A SEN LSA however was supposed to be in that classroom for one child only.
However - due to larger classes and less TAs my job as a SEN TA was undermined constantly by the demands of some teachers (not all) wanting me to help run the class or take large groups (usually the less able pupils) out of class to do an activity elsewhere with my poor SEN child 'tagging along' :(
This last point, i'd like to add, may go some way to explain how the misuse of TAs can lead to a decline in educational standards, as mentioned above.
I underwent training regularly for the first 5 years of my job. Then funding was never available to send me. I was told if i wanted more training i would have to pay myself. The last course i couldn't do would have cost me £500. All the while my contract stated that i should handle no more than 7 children in one group and should not be expected to replace the teacher for main lessons. Yet time and time again i was sent off out of class with 8/9/10 of the least able/rowdy children, to any room i could find, and be expected to teach them the basics of the lesson going on in the classroom.
Over and over again TAs leave and are not replaced due to funding. More and more pressure on the staff remaining.
Many times i would raise the point with the head that i was being put in a position as a SEN TA which undermined my ability to carry out the target work for SEN child. The answers would always be budget related with the heavy implication that i knew where the door was if i couldn't cope.
With lack of training there is no investment in the TAs anymore and therefore little value attached to them. There were always hordes of parents waiting to jump into any role that became vacant.
It's not right. But it was the sad reality of TA/SEN work for me.