on entering the workshop he found many students worked there

原句中应该是found many students working there 吧
进入工厂的时候,他发现有很多学生再哪儿工作.
on 用来限定某一个较短的时间（通常为一日,比如 on Aug.20）,或动作的某一个瞬间(如题),翻译成“在...”