Teachers can be very hit or miss, especially where it comes to tenured teachers, as
@Judge points out. It sounds like you got a bad teacher - I'm a teacher myself and I know that students have different learning styles, all go at different paces and what works for one student might not work for others. And if he talks too fast, doesn't things clearly and losing students, then he needs to address this. In an ideal world, this would happen, but unfortuately many teachers are either indifferent or incompetant, they just do what is necessary to be paid and won't make the extra effort.
Solutions to this problem might be to try to approach that teacher when he is on his own to go through things once more with you, write a list of questions you want to ask or points you didn't understand. Another solution would be to askd a fellow student to help you. You could also talk to your parents about the possibility of having private tuition, or you could research the topic yourself, use onlines sites, YouTube tutorials or self help books. You could even make a thread here and ask, I'm sure there are plenty of people who would be willing to help.
At school, I had bad teachers and good ones, as I said it was very hit or miss. My maths teacher was excellent and really helped me - she went round all the students individually in class and checked that they understood the material, she also wrote very clear examples on the board and didn't rely only on oral presentation alone, but made visual diagrams. She would also explain things after class individually if asked. Not so the English teacher. It was sink or swim; either you got it - you got her, or if you didn't, she wouldn't help you. My Asperger's was undiagnosed and many of my teachers didn't like me, didn't help me when I was struggling, had no patience with me. An official diagnosis and autism card would have been useful in many circumstances.
If you have an autism card, your teachers might take your difficulties more seriously, but they still might not know what exactly autism is or how it affects you (it is, after all, a spectrum), so I would write down exactly what difficulties you have on a card and keep that with you, too. Especially the SPD.