ALTs (Assistant Language Teachers) mostly work in Elementary and Junior High School as an assistant to the main JTE (Japanese Teacher of English). Most of the time the JTE will explain the grammar in Japanese so the kids have a better understanding but can put you on the spot with a grammatical question. I recommend teaching in Elementary or JHS with an ALT company as this was the most rewarding experience for me and you get the chance to participate in real school life with real students. I would highly recommend contacting ALTIA Central Japan if you are interested in working as an ALT in Japan as they were the best company in Japan for ALTs.
Whatever teaching you do it will pay to have a good understanding of English grammar.
How to become an English Conversation Teacher
Working in Japan