This is the 34th most frequent Slovenian word.
niso
"Are not."
In this sentence, 'niso' is used as the third person plural negative form of the verb 'to be' (so biti) in Slovenian, combined with the past participle 'pozabili' to form the negative past tense.
Ljudje niso pozabili na dogodek.
People have not forgotten about the event.
Here, 'niso' negates the statement about 'everyone' (vsih) coming to the party, expressing that the group was not complete.
Niso vsi prišli na zabavo.
Not everyone came to the party.
In this case, 'niso' serves to create a negative verb form in the present tense concerning the subject 'students'.