Teaching at is one of the most meaningful volunteer opportunities in Nepal. Nepal is a developing country and the education level is not that developed. The way children learn and teachers teach in Nepal is completely different from other countries. If you are good teacher, you can help with teacher development, teaching teachers to better themselves to work with students and make them learn what they are taught. 
You can teach English, Science, Math, Accounting or whichever subject you are comfortable with. If you speak English well but haven’t taught any classes in your life, don’t worry your conversation and interaction with children could be the way for them to learn proper spoken English. Getting involved with teaching this way is more of a cultural exchange and children learn the way life differs in your country from that in Nepal.