the boy likes playing jokes on his classmates,接下来为

the boy likes playing jokes on his classmates,which of course makes him unpopular
这里的which是指代前面整个句子the boy likes playing jokes on his classmates,
男孩喜欢抓弄别人,这当然会让他不受欢迎.
which指的是整个前面,是非限制性定语从句
如果想用上for, 应该是
the boy likes playing jokes on his classmates,for which he is unpopular
for which he is unpopular 还原是he is unpopular for sth.
那男孩喜欢抓弄别人,这也是他不受欢迎的原因
for 表示因为,由于这原因, which 还是指代前面的~