Ok, seems like our mandarin-speaking comrades have found out the harem route.
Harem route can only be done on 2nd play-through, and it requires you to max out the relationship with only one girl first.
Each positive interaction gives +3 to the count, out of 100. Not sure what relationship points you start at as student (normal mode), or teacher mode (hard mode).
When a girl relationship gets to the following points, you can do the following:
Recognize you as a "boy" friend - Handjob (After you confess)
Like you? - Blowjob, more breast play etc
Like you! - It's sex-in time!
Once you get your first girl to max relationship, continue and start your 2nd play-through. You will be given 3 prompts, and select the 2nd choice to bring over the relationship count.
On the 2nd play-through, you need to confess to the first girl again, then you can start building relationship with the other girl. After you build enough relationship to confess to the second girl, choose the left choice as usual. Congrats, you are now in harem mode.