I will be starting as a high school maths teacher next year. I really really wish I knew how to fix students who arrive in year N and haven't learned the critical skills they were supposed to have mastered at the end of year N-1. Teachers don't choose curricula, and curricula are too dense for mainstream students. We already have more than one semester's worth of material to get through in a semester.
If a bunch of students arrive in my class in February, and the mid year exam is in June, and they don't know any of the stuff they need for this year, how do I fit all the extra material in? This is not a rhetorical question!
First, relax and realize you can't fix everything. That doesn't mean you shouldn't try, but you need to set realistic goals or you'll burn yourself out really quickly. The students really need more lesson time, and more practice time. If the student has resources you can refer them to one of the many tutoring centers designed to address this sort of thing. If they don't have resources, most attempts at solutions will involve a lot of your personal time, which will already be highly strained as a new teacher.
Ask the other highschool teachers you work ...
Post by fellow LW reader Razib Khan, who many here probably know from the gnxp site or perhaps from his debate with Eliezer.