Ghost- Possessed Varun. Then possessed some of the BFFs and stole their memories, and tried to fabricate threats. Also may have possessed Prithvi and Anu, and otherwise, taken the form of the mystery man. Also may have possessed one student inside the girls' change room, and stayed invisible. The BFFs may have seen a ghost in some form there. By the way, Ella's mom is played by an actress who played a vampire in a show full of vampires and werewolves.
Alien- we can keep guessing what these can do. Maybe possess humans like ghosts (or that black costume in the Spider-Man franchise). Possibly downloaded and hacked personal information about the BFFs using its technology- and of course, may have translated its words and thoughts into the local language. The party site may be an alien base. Anu and Prithvi are either aliens or spies chasing aliens.
Govermnent conspiracy- India's own Area 51, Hangar 18. They've hacked into the BFF's accounts and used the information they found, against them, so that the BFFs don't uncover their secret. Kidnapped Varun and kept him in cold storage. Anu and Prithvi are spies tailing the BFFs. As also the mystery man. He could even be Varun on steroids and with plastic surgery- done by that group. And also, as a Government agent, able to avoid police action for speeding and rash driving.
Then the sequence of events. If someone is imagining everything, who is it? Varun, planning his 'revenge'? Sanju, feeling 'guilty'?
RGV threatens "every dog has its day"- dogs barking are a motif- maybe one recording came useful? Which one was it?
Just what was that case Rigved was talking about?
Vinnie gets a piece with R on the back, and she is nuts about Prof Rigved. Ella gets the piece with P, and we see her getting close to Prithvi. Just who is this Mr I that Sanju may get stuck with?
And the 'secret'- was it something they did to Varun? Was it something unbelievable that happened to Varun? Or was it someone else's secret they picked up?