From your choices Manali would be the best for a never forgetting experience Shimla an Nanital is a little bit overhyped so go for Manali to make a desire of exploring more and more.
If you have already been to Manali, try somewhere else. Shimla and Nainital both are small and crowded. Go to Tirthan valley or Parvati valley. You can also go to Dharamshala and Mcleodganj
Hello Guchhi, i would prefer Shimla and Manali as its more vibrant and you have more things to visit and do here compared to Nainital. As you are going with your school friends its better you visit some vibrant and happening place.
U should give a try to nanital, becoj on this route u can also explore jim corbett park(jungle safari-a living experince), then u can go further to Rani khet.
Hi If you have to choose from these 3 then go to Shimla and cover nearby areas- Narkanda, Kufri, Chail, Theog, Mashobra
Other places where you can go:
1. Auli
2. Chopta
3. Munsiyari
4. Binsar
5. Nagtibba
6. Kanatal
7. Dhanaulti
8. Valley of Flowers
9. Mana Village
10. Chamba