It is better to get a  visa to study ini Korea even though it is just for 1 semester. You can issue C-3(90days) visas at Korean Embassies in 1 week, and visas can be extended and changed according to the student's nationality and other information. However, currently the immigration office does not issue C-3 visas for Chinese students, so Chinese students must apply for D-4(6months) visas at the immigration office.