When and how should I ask her?

I have had a crush on this girl im my school for 5 years and I want to asker out to have lunch with me like in the school somewhere cuz thats more casual and not as intimidating for either of us cuz we are both shy. should I ask her at her locker between classes to eat lunch with me that day or ask her when I walk home (we walk like the same way for 10 min) to meet up the next day? If I ask her at her locker its crowded and non-private but if I ask her after school then I have to dich my friends that I walk with just to walk with her on the other side of the street.(my friends a really imateur and pretty much think all girls still have cooties. they would be like WTF are you doing) BTW I know she likes me and I know she knows I like her but we have never talk cuz we are too shy. when should I ask her and how should I approach her in a non-intimidating way so its easy for both of us?