When talking with a local flight school about my interest in starting training, I started asking questions about how the AME process works. The flight instructor then asked me if I had any specific medical issues. Are you required to disclose any health information to your flight instructor? What about if the issue(s) could be disqualifying? Would you get in trouble for lying to the instructor (to avoid judgement or maintain privacy) but NOT lying during the AME?
When it comes to the AME, you must disclose everything fully and completely. Failure to do so may void any license and insurance and expose you to extraordinary civil and criminal risk.
When it comes to your flight instructor, who is not a medical professional, you do not need to disclose the specifics of your situation. Saying, "I have concerns about passing a Class III Medical Exam due to certain conditions" is sufficient, and is not the same thing as lying. Do not claim you are healthy when you know you are not.
Your instructor may decide not to continue training until a physical is passed, or he may decide just to be a little extra vigilant during training, and delay certain elements or maneuvers depending on the information you choose to provide.
For example: If you choose to disclose vertigo problems, he may avoid steep turns until medical clearance. If you choose to disclose vision issues, he may avoid night-flight.
Regardless, you will need a medical before your first solo flight.
Do not lie to your instructor, but that is not the same thing as disclosing full details of your health with a non-medic.
There's no legal or regulatory obligation to tell your instructor about medical conditions as others have said. There may be rare cases where there are insurance considerations the flight school must take into account, in which case they should explain that.
There's good reasons to be open, however: - Not disclosing a medical condition could increase the risk to both you and your instructor, depending on what that is. The more the instructor knows about you the more they can look out for the specific issues a medical issue may raise - Training works better if there is a rapport and trust between the instructor and student. Lying to an instructor is a breach of trust, and could damage the relationship.
Think of it this way: how would you like it if you were an instructor and found out a student wasn't telling you he/she was prone to panic attacks? And you found out on final approach when the student suddenly went rigid, or worse pulled back on the controls and put you both in a life threatening situation? You'd want to know so you could make an informed choice on whether to take that student on, and if you did you'd probably elect a different training regime, one with more gradual steps.
There is no obligation to disclose anything to your CFI, just your AME. That said, your CFI may have useful advice about common problems, if you're willing to talk about them, and they may suggest delaying relevant parts of your training until thosr problems are resolved by your AME.
If a CFI (or anyone besides your AME) asks a question you don't want to answer, then tell them that, rather than evading the question or lying (even if only by omission), and they should respect your privacy. Just knowing there's something they don't know is good enough.
If your flight school requires medical forms, then IMHO, find a new one. All they need to know is whether or not you have your medical certificate.
At this stage, when you are just talking about maybe starting to learn to fly, this person is not acting as your flight instructor. He's just a person giving you information about a process. You aren't under any legal obligation to disclose anything to him.
On the other hand, flight instructors often have a decent knowledge of the medical requirements to fly, and might have been able to give you a good opinion on whether any particular condition is disqualifying.
At a later stage, when you are actually taking instruction, it's probably a bad idea to withhold useful information from your instructor. It is likely that a flight school will make you fill in a form which asks about medical conditions, and lying on that would be a very bad idea.
The reason he asked that is because he thinks you asked an XY question, and thinks the "X" is "I have a medical problem that I fear might fail me on my medical". So he is asking only to answer your query, and he's not wasting time on the Y, and taking the shorter path to X. Not his first rodeo...
As such, he's listening for relevant medical concerns likely to fail you, i.e. That could affect your safety to fly. It would be an "overshare" to mention your excema, HIV status, enlarged prostate, well-managed cancer, eidetic memory, gluten allergy, or need for Viagra.
So answer the X: say why you think you'd have trouble with the medical.
You can always trot out the old "asking for a friend"... But then, expect to have an instructor who takes precautions against you having that problem in flight with him. Really, a lying relationship is not one you want to have in the cockpit of a plane.