How much is a soft sleeper ticket to Beijing and when is school holiday resumes

You mean Nanjing Road in Shanghai? End of August-early September is school holiday resumes. Trains ticket to Beijing need booking several day in advance all the time. So, suggest you book it early. As to the ticket price, I hope this website can tell you, for I am not sure where you are leaving.
https://www.travelchinaguide.com/china-trains/