How to decline answering details about my thesis?

The one valid point I can see here(*) for not sharing the state of the thesis:

For the research, I am visiting classrooms at various public schools.

You should not share the preliminary details of your thesis if this could affect the outcome of your study.
As I understood the question, the school (or some teachers or students) may be the subjects you study, so they should be as little influenced by you as possible (that is, don't tell them that they are in the placebo group).

(*) things would be different e.g. if you were in an engineering field and working towards a patent

I'm actually a bit surprised by the harsh comments here. I do come from Computer Science, so the situation might be a bit different.

But still, I know more than one student with advisers that explicitly prohibited presenting ongoing unpublished work. Sometimes it is a bit more relaxed (e.g. it is okay to present it to the people in your team or lab), but in general, it is not uncommon in my area. The explanation I got once is that if a senior researcher with a lot of resources recognized an idea as potentially good and gives it to multiple junior students to work on, he can develop the idea faster than the "original idea holders" (i.e. 1 PhD student + his/her adviser).

On the other hand, I personally believe that the whole point of research is to expand and share knowledge, so I frown upon being too protective of one's work. Also, putting a date on your presentation, and making it available online, diminishes the chances of stealing.

That being said, I think offering a 90-minute presentation, with discussion, based at least partially on your work, is a very nice offer. The only reason I can think of why they would not be pleased is that they expected a talk on some specific topic.

Some suggestions about what you could do as an alternative:

• prepare an in-depth exhaustive talk about just one problem/theory from your thesis, preferably something already published in an article
• prepare a talk about motivation for your theories, problems that dictated the research direction and reserve some time for a discussion session, where you do offer your own theories as solutions, but other attendees are expected to participate as well.

• offer to come back after finishing your thesis, and presenting everything in-depth (possibly over several sessions) when all the work is finished, published and peer reviewed (see both my points below)

  This should give you enough flexibility to avoid talking about any sensitive areas.

Also, some "guarding" explanations that seem quite reasonable to me:

• a PhD thesis is a 3+ year long effort, often a compilation of many works and ideas by the author. Even though it is centered around one subject, it probably deals with several issues, approaches, angles...

  All in all, it is a lot of material. Explaining everything is something what is expected at a PhD defense: and even then, in that 1+ hour talk, some students can't cover all their work.

  Thus, explaining that presenting everything simply requires too much time, and offering the school to choose a issue you deal with that is of particular interest, does not seem bad or disrespectful or overly protective.

• if you are against disclosing details of your ongoing work, you should be able to come up with some legitimate reasons. If the work is ongoing and unpublished, you can argue that you do not want to present ideas that did not go through peer-review process.

  Peer-review is the process that (should) ensure excellence and relevance of published research. Judging the relevance and correctness of your own work is something not left to the author, and it should be fine if you do not want to present something in detail that is not yet accepted by academic community.

  But, even in this case, and after such an explanation, offering to include some of this work as a smaller part of your presentation (e.g. ideas for future work, or discussion), with a beforehand explanation that those are just discussable, underdeveloped ideas and should be treated as such.

Bottom line is, it is not acceptable to guard your research just because others do it. But, if you look at why others do it and understand their reasons, you can decide whether that reasoning applies to you and your work.

Is it acceptable practice for graduate students to be guarding of their work?

Without saying acceptable to whom it should be, you won't get any good answer. Is that acceptable to you? Is it acceptable to your adviser? Is such a behaviour acceptable in the context of your "contract" with the subject of your research, namely the school you speak about?

Is this common enough that I can cite it as a reason for wanting to avoid sharing the details?

This would be perhaps a bit field-specific, but as far as I can say, this is not a common practice, nor I would accept it if somebody would use it as an argument. In my opinion, this stance is very much against the purpose of publicly-funded research.

First of all, realize, doing research has a highly ideal objective: generate, advance and maintain knowledge of humankind. If this does not appeal to you, then ask yourself who pays for your research. If it is general public, i.e., taxpayers of the country you reside in, then they deserve to see your results upon request. If it is so, that includes the officials of the school in question. There is your answer.

Secondly, what is the benefit of you keeping the results private? Are you worried about being scooped? On that, see elsewhere on this site and build an opinion for yourself whether that is a real threat.

Finally, as a graduate student, you should be rather happy that somebody is interested in your research. Most of your peers do not get that kind of attention at all.

To sum up: Unless you are paid from a resource which contractually forbids you to disclose your research results, there is no "objective" reason to refuse sharing your results. It will of course remain your freedom not to share them, but in that case you must stand up to your decision and attribute the refusal to your own personal decision without hiding yourself behind some "common practice". Take responsibility for your decisions!