About 7,000 South Korean students held nationwide protests in advance of the seventh anniversary Monday of a bloody anti-government uprising in Kwangju and again challenged President Chun Doo Hwan to allow the direct election of his successor. A national police spokesman said the students demonstrated on 21 campuses in Seoul and in provincial cities including Kwangju and that at least 39 youths were detained for questioning. Press reports said five students and 17 policemen were injured in campus clashes. Chun decided April 13 to reject any electoral change. Troops put down the 1980 uprising in nine days, killing at least 200 people.