As Ethel says, it depends very much on the school, the precise nature of your job and your expected relationship with the youngsters and, to some extent, your preference.
When I started as TA in a special school (secondary), I was told that all the teachers were Mr, Mrs etc., but it was up to TAs as to what they preferred the kids to call them - they just expected the kids to respect their choice. I chose my first name, partly because I was also the school's youth worker 'out of hours' and it was less confusing for them.
The children in the primary phase were always encouraged to use Mr or Mrs, and most could understand the difference by the time they were looking forward to moving up senior school.
I would be guided by staff at the school. I'm sure they won't mind you asking.