Being a person who always process the visa's business,I don't suggest you to leave secretly.I know how long it will take to get your visa's documents done.Sometimes it takes short time,sometimes it takes long time.It all depends on the government.ALL the documents have to go through the two departments of the government.Short time could be done in one week,but if the officer go to do business to other province,your documents possible will stay in their office for a few weeks,so if you wait for a month,it is very normal.
But if you don't like the children's classes,no matter how good they said,you should not take it,if you have already taken the offer,then you should talk to them very frankly that you are not capable of teaching the children courses.And you prefer to end the contract.The contract is made for both parties,you have right to ask to finish the contract if you have good reason.
Leaving secretly is not a good and professional way to solve the problem,it may cause lots of problem such as no school will and could hire you.
That is my suggestion.