EDITOR #11:
I understand your apprehension. In high school, it’s easy to walk into awkward situations and that is presumably why you hesitate to tell your friend you like her.
Ask yourself why you like this girl. Have you known her for a long time? If you have, then you should be comfortable enough around her that telling her how you feel is not awkward. If you haven’t known her for very long, you should acquaint yourself better with her to make sure you legitimately are interested in her and not just telling yourself falsities.
Have you ever contacted your friend outside of school (or wherever you usually see her)? If you haven’t, you should host a small event, like a movie night, and invite some of your close friends. The list should include both guys and girls, a good balance. But most importantly, invite the crush! If she cannot come, do not feel disappointed and make negative assumptions. If the event was fun enough, you could always hold another one at a time when she is available.
As for when to admit your feelings—I cannot tell you the exact time when to do it. Like I mentioned earlier, you should be comfortable enough with your friend that telling her will be natural.
Good luck!