I have noticed that some schools ask about expunged records, and yet most states have a policy of being able to legally say "no" to questions about convictions if expunged. How can it be legal to backdoor it like that?
Things aren't illegal "just because". They're illegal because there's a law making them illegal. Can you point to a specific piece of legislation that says that it is illegal to ask about expunged convictions?
If not, it's not illegal. Simple as that. We don't have proscriptive law here (meaning the state doesn't tell us what we CAN do.)
The state has to allow any forms of conduct that they don't specifically prohibit.
What they have told you is that when asked if you have been convicted, that you do not need to divulge.
However, they haven't said that for special purposes (a security clearance background check, applying to work at sensitive facilities, to join the military, to apply to the state's bar) that you cannot be asked.
You can connect the dots yourself on this one. If it's not illegal to ask, and they'll hold it against you if you don't answer truthfully, you need to respond accordingly.
This reminds me of a guy who came into my office asking about an application. As we were talking, I mentioned that we run background checks and he needs a clean driving record and no history of felonies. He claimed that it was ILLEGAL to discriminate against him because he served his time and now he's out. I told him that's an interesting theory, but that he wasn't eligible to work in my business. He demanded to see documentation of this policy and wanted to know how it was legal.
Now, more germane to your situation, if the expungement was for something pretty minor (like, say, a juvenile misdemeanor), then I doubt you have much to worry about. You can probably get a Top Secret clearance in the military and get admitted to the state bar or whatever, provided you show them that your juvenile indescretions are not indicative of the person you are, today. One way you show them this is by answering their current questions fully and honestly.
Best of luck.