While some students were asked to pay rs 1000 each, few others were ordered to pay rs 250. Some of the students who apologised for speaking in Malayalam escaped from the fine.
The incident occurred at the Holy Grace English Medium Higher Secondary school at Kuruvilassery in Mala, police said.
The CBSE school is managed by a society of 30 persons. There are 70 students studying in Plus One and Plus Two (11th and 12th standards respectively).
The school had stipulated that students should converse only in English and not Malayalam, management sources said.
The students were not allowed to sit in the classroom on Thursday. Protesting against the punishment, some students said they hailed from poor families and would not be able to pay the fine.