The official word is that unlocking the form would not delay an interview. However, we tend to think that it depends on the information you changing in DS-260. I would think that adding high school would not delay the interview. So unlock it, make the changes, double check everything and re-submit right away. KCC should go by your original submission date.
What's your region (Africa, Asia, Europe)?
There is nothing to worry about. If you are from Africa or Europe, you will get your chance even if it sightly delays your processing. If you are from Asia, your interview is about a year away, so there is plenty of time.
Thank you so much!
I'm from European region, Moscow embassy. As I see on another (russian speaking) forum my interview will be approximately in december if unlocking will not affect this.
Well, now one reason less to worry about