All the studens are busy (study)for the English test

studying
be busy doing sth.
be busy with sth.
这里是 所有的学生都忙着为英语测试学习.

所有的学生都忙着准备英文考试

所有同学都忙于英文测验。
考试则更正式，用exam

for 改为with .

studying，be busy in dong sth ,in 可以省略