I don't think it is grammatically incorrect but too many S's might sound weird. It is recommended to use :
"Many students got a headache while learning/studying PMIT"
"Most of the students got a headache while learning/studying PMIT"
"Bulk" is mostly used for non living things. It can be used for students but doesn't sound too natural. It can however be used to magnify the numbers, i.e. in this case bulk gives a feeling that too many students find it difficult to study PMIT.