but I have a very difficult class, some of the children are put into additional social skills lessons. If I accept the mentoring and stay on my idea is that, these children, the same children will not change in 6 weeks
But that is very normal and typical. The school Managers are offering help to you, to master something they would probably have expected you to learn to do during your degree or Post -grad training, and then the two years of ECT you have completed. Part of the skill - many would argue the most important part - is how you relate to the children in front of you, whatever challenges they present to you. Yes, it is very challenging at first, but you are now in your 3rd year.
The workload is already too much for me. Even appraisal meetings are scheduled during our lunch times and after school.
Er... I'm confused by this comment, coming from someone who is a teacher, so has done teaching pracs and two years of work before this year. When did you think teachers hold their meetings ? 
They would like me to plan maths and I said I would try of-course but then they would like me to sit with deputy head after school, in my personal time.
Again, "after school" is worktime. As a person in their 3rd year of teaching, I expect when they employed you, they would assume you could do this anyway. However, having found you can't, the DHT is now giving up some of their time, to offer you some support, and you are somehow thinking this is unfair of the school ??? 
Marking, we teach least 3/4 subjects a day and that is 30 x 4 books to mark daily. They want every single work to be marked
You've had this answered a lot, on both threads. In KS1, you don't have piles of books to mark. You mark as you move round the room, during the lessons. You make sure the dc leave their books open, so when you do look through a pile, they are all there, ready at the right page. You do a variety of all sorts of learning and recording of work, so there aren't 'books to mark' from every lesson. Plenty of the lessons you teach won't have books - PE, music, art, and then lots of activities from Foundation subjects and Core subjects.