Hardly had the bell rung / when the students / rushed out of the class. / No error

'Hardly' is correctly paired with an inverted past perfect and followed by 'when'. The simple past 'rushed' in the second clause is also correct. Since all parts conform to the pattern, there is no error. The trap is to suspect the inversion or the use of 'when'. Fixed patterns such as hardly...when and no sooner...than must be memorised along with the tense and inversion they demand.