Oh, boy, have I dealt with that! Our youngest child (whom I call Sage on MDC) has Asperger's and Tourette Syndrome. I think 86% of the questions that were asked of her were answered "I don't know" from about 10 until... well she still does this, but not as much. She'll be 14 in a few days.
My Sage is shy. I guess that's what it is. People are actually lucky to get an "I don't know." from her because Sage is just as likely to stare into space (I really think she's doing Integral Calculus in her head, but I'm not sure) or not make eye contact, or simply look at me and wait for me to answer. She doesn't have ADHD, but our oldest one does, and oddly, she never answered "I don't know" but often people who asked (ask) her a question would be subjected to details of every random thought that flitted through her overwrought mind in the last few days. Every... Random....Thought...... I actually prefer "I don't know."
Kids who are neuro atypical do this a lot. I do try to ask Sage the same question and say things like, "Mr. R wanted to know how you are liking school this year. Could you look at him and maybe let him know how you are liking it, or not?" (Because usually "How's school going this year?" is about the only thing a lot of adults can think to ask most pre-teens) Sometimes she would quietly say, "It's OK." or, "Not as fun as last year." (but that might promote more questions, so she would usually just say, "OK.") If she isn't feeling well or the Tourette is really bad she is more likely to not make eye contact and not answer people.
I realize I haven't given any solutions. I don't know if there are any. Yeah. Some people think it's "rude." But, most of the time, they don't really care if my dd is "liking school" or not, they just notice her there (because she's usually standing right next to me slightly behind me) and feel like they need to say something. Sage and kids like her are actually very comfortable in non-verbal silence, it's just other people who may find this unnerving.
I'm a talker. However, Sage has taught me (being nearly 38 when she was born and nearly 48 when she started showing this quality in true form) that we don't have to keep up constant patter just because we are in the proximity of an other human. Her father (my DH, also Aspies) has been trying to teach me this for.... at least 32 years
I will often try to "apologize" when Sage doesn't answer people by saying, "She's kind of shy" which then prompts Sage to say, "NO I'm not!" *sigh* (She is.) People do understand "shy" and I often tell her directly, "I don't want you to think I am talking about you like you aren't here, but when people want a response, it's the best I can do to save feelings to say you are shy." Sage is beginning to understand the need to "save people's feelings" (something her Papa has yet to learn if he doesn't feel like talking) but she's still "quiet" most of the time.
The hardest part is when people give her a choice of something. She has Sensory Processing Disorder and only eats a few foods, when people ask her "Do you want this, this or this? ) and the last one is the ONLY one she will eat and she says, "I don't know." and she is then given something outside of her Insisted Upon Food Group, eventually she is learning to actually speak up for herself or get stuck with something like Sloppy Joes or food she would never eat in a million years.
Nope. I don't have any answers. I think the "She's kind of shy." is best for the time being. Your child will talk when she gets older (maybe) perhaps she doesn't feel anything particular about what people are asking her, perhaps it's difficult for her to organize her thoughts, so she'd rather not say anything right away (my oldest couldn't organize her thoughts so would just jump in before her brain was in gear and ramble until she struck upon something.... I'm kind of like that, too.) You probably don't want people to think she is "rude" but to her she isn't being rude she's a preteen who may need to organize her thoughts, may not feel like talking and may just not want to have her thoughts interrupted.
Good luck. And remember, to her, she may recognize that the things people are asking her are for the most part, empty words that people use to fill spaces.... and she doesn't want to deal with that. She'll probably eventually learn social protocol, but it may take some time.