What exactly do you mean by "use as a source"?

If you mean that you'll refer to it for more information or that you'll quote a short part of it (properly attributed), I think* there is no problem.

If you mean to use it to back up a claim you're making, it depends on the quality of the response you're using a source (some comments are very high quality, others less so). It also depends on whether or not your claim is properly supported in your source. Make sure you're not relying on an off-hand comment in an otherwise well-sourced answer. It would probably be best if you track down and refer to the original source (book/paper) though; if the comment you're using as a source is high quality its sources will be listed.

If you mean to "build upon" it by copying a part of it and editing it, that's a no-no, in my opinion*. Your response should be entirely your work, with perhaps a few short and properly attributed quotes. (Note that this does not just apply to responses on this sub, it applies to all your sources, and all your historical works.)

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* I'm being clear that these are my opinions, because I don't decide what is and what is not "permissible". I do believe my opinions on this subject are shared by most historians.

(edit: typo)

It sounds like you are unclear about what a source is and why it matters. It largely depends on the context, i.e. what you intend to do with them, what you are trying to prove. I'm not an historian, however the principles are the same for any type of skeptical enquiry. I invite you to read Wikipedia's policy on sources; theirs is not a policy applicable to all contexts, however it's a good example.

Primary sources are the original documents or artifacts, or photos, copies and translations thereof. In science they are (non review) journal articles. They may be used on WP but only with care because they may need a specialist with a good background to be properly understood, if at all.

WP favors reliable secondary sources. That means expert opinion based on primary sources, including but not limited to review articles. Typically for history a book written by a historian specialized in the field. This is the preferred source for WP. If primary sources are used on WP, they are usually there to complement the secondary sources that refer to them.

Tertiary sources are one step removed still and are usually things like encyclopedias themselves or introductory textbooks. They are usually not great sources for the purpose of writing even a school dissertation, in that you'd better not cite Wikipedia or Encyclopedia Britannica as such, but rather cite the secondary and primary sources they themselves cite.

The categories are not entirely clear cut. For example a contemporary news article about WWI cannot really be considered a secondary source for WP's purposes because it requires expert knowledge to sift through the propaganda and fog of war. It's more like a primary source yet it was in all likelihood written by someone far removed from the action and with a particular bias.

So, to answer your question, if you want to use answers in this subreddit in a school work, you should probably only do so as a starting point, read the (usually secondary) source material referred to, and cite that as the source. I guess it wouldn't hurt to mention /r/askhistorians as how you found it, but that's probably neither necessary nor expected.