Help me if you can. How to progress with Helena? Without her, Claire is impossible and Helena says talk with Mila in school. But I get only two options there. Work with Mila or leave. Is there any particular day where I can see Mila out of class room?
No there isn't another time at school I don't believe, but actually it's funny you bring up Mila too because that's part of your answer! So at some point in the game, Claire will come over to spend the night at your house with Zoe; in order to get this to happen, you have to be far enough with both Zoe and Claire (you'll know because by then they'll have gotten in a few arguments). If you're already past this point, great.
Make sure you keep progressing with both Zoe and Claire's stories up until ANOTHER sleep over at your house (make sure you visit them in Zoe's room when these events are occurring too). At some point, Helena will come over to either drop Claire off or pick her up (I don't remember) from your house (this might be during the first sleepover). Then during the NEXT sleepover she'll go out with Mila on a "girl's night out". If you've already seen these events, GREAT (if not, get them done)!
Now when it comes to Mila, you might have to speak to her a few times, but at some point if I remember correctly she'll ask you to COME WITH her and Helena for their next "girl's night out"... it is in this event that Helena's path really starts. I won't spoil it, but long story short, the 2 of you meet up in the bathroom

... you just have to make sure that you're far enough with both Zoe and Claire, and that you've gone on this night out with Mila and Helena. If you haven't done this event yet and are wondering how to trigger it, I'd suggest talking with Mila, Zoe, and Claire at EVERY time of the day individually until Mila asks you to go with her.
If you ARE past this event, then my next suggestion would be to try and talk to Claire until you have permission to visit her house. When you can do that, talk to Helena in her living room at each time of the day.
So long story short, make sure Zoe and Claire are friends, make sure they've had their sleepovers, make sure your stats with all 4 of them are as maxed out as possible, and make sure you talk to all of them whenever possible. If none of this bullshit worked, @ me again and I'll figure it out for sure

. Hope this helped!