My Chemistry teacher left mid-year, resulting in weeks of free chemistry! We couldn't really move on as our next topic was more complex. You can definetely teach yourself the theory part of chemistry but you might need help on mathemetical side of chem. If you are in 5th year and finding chem really difficult than you can consider dropping it, however I dont recommend it! If your in 6th year, than dont even think about dropping chem. Just do exam questions daily and keep practicing.
Judging by the description of your teacher, he/she seems to be careless about the students, so there is no harm in confronting the teacher and telling he/she to get your act together. Obviously in a more professional manner. Mabye even tell them to switch up the teaching style as you find it difficult to keep up.