So one of the things that come to mind is the plot inconsistency, especially when it comes to character ages. Like remember when Vibha was with her friends, didn't they say something like one of them were getting married? UM. If Vibha was in the same batch as Kaisi Yeh Yaariyan's Fab 5, wouldn't they all be the same age? So how are Vibha's friends' getting married and managing businesses if they're all from the same year (as you know Vibha only spent one year at Warrior High) and therefore they should all be about 19?
I do think that Vibha's age was rectified as the show went on; I think it was the episode before she is interviewed by the principle, the precap showed Vibha saying she was from batch 2010 (making her older than she is now) and yet when the episode came, it was edited to Vibha saying she's from batch 2013. This editing does mean there was a blooper (if you've noticed) in the file that the principle was reading where the certificate or whatever for somewhere she went to study dating 2011'. A complete blooper seeing as she was at school during 2011, and went to Warrior High in 2013. (of course if she was in batch 2010 then that blooper would not have been a blooper).
But anyway. The friend scene and blooper scene aside, Vibha is (as she has mentioned again and again) from batch 2013 making her the same ages as the Yaariyan kids who have been shown as 19. I'm actually okay with Vibha being a part of Fab 5 (I don't mean in their band but just mean their friend) pre-Yaariyan days because it would make her much younger than I thought she would be (you know if the writers are planning for a ParthxVibha ship, he's 16 and she's 19 so they're more closer in age than when I thought of her as being at least 22. Although, I'm still not sure about the teacher-student relationship).
So I was thinking about this and thought it makes much more sense now. If I consider it from my perspective (as in British schooling system age thingy), Sid, Krisann and the others who know each other pre-Warrior High were probably at the secondary school' side of Warrior High. Basically where I come from, sometimes schools are separated in the actual secondary school (so ages 10 going on 11 up to ages 15 going on 16), and then there is the sixth form section (ages 16 going onto 17 up to 18). Of course there is also separate colleges' (which is just the sixth form section of a school). Does that make any sense?
Anyway. So. Sid, Krisann etc attended Warrior High's school from where they are now at the new side of Warrior High, where they are the juniors (ie, first years of college). The seniors are probably a year older than them. The kids at Warrior High are currently 16 but will be 17 in this first year, they will then turn 18 in their second year. That is, if Warrior High only has two years in their curriculum. And then if we consider this to Yaariyan - say the kids from Warrior High went to SPACE, their first year (which I guess Nandini and Navya are in although their ages haven't been told yet) will be them turning from 18 to 19. Fab 5 are obviously 19 right now however seeing as they are in their second year (from my logic) they're be turning 20 in that year.
That means Vibha started as a 16 year old at Warrior High where she met Fab 5; [Harshad is not with them because he's like a year (maybe) older than Alya and would have been their senior' (ps, I'm also guessing the Fab 5 video scene was before their fallout with Harshad) so unless Harshad has failed his year at SPACE (where he'd then have to retake the year and therefore would be in Fab 5's year), he'd be 20 going on 21]. ANYWAY. She left after a year so when she probably turned 17, and then spent the next two years away' and is now back at Warrior High. Again if I think it from where I come from, even if Vibha hasn't gone to University (basically say where Fab 5 are at right now), and even if she hadn't had further education, she can still get a job. She's got the basics so she can teach as an assistant teacher (which she is doing now) but not an actual teacher (which she wanted to apply for?).