Student visa single entry for 1 year stay

I will be studying in Japan for 6 months with a single entry student visa for 1 year stay. If i dont plan to return to my home country within the 6 months period, it will not cause any problems for when i finished my study and want to return home 6 months later, right?

When you are not planning to enroll a school after your 6 months you need to change the visa.
Having a visa means you need to follow the requirements of the visa, and one of the requirements is studying at a school. If you are not changing your visa to a correct visa you can have problems with immigration if they find out. Technically the school can/or will inform immigration when you quite your study.

If your study has finished but you want to remain in Japan for a little longer, your best option is to convert it to a tourist visa. To do this, simply go to the immigration office, and tell them that you want to do this. They will require a reason, but saying something like you want to travel around Japan before going home should be satisfactory.