Don't start by trying to prove she's exaggerating or that it's laziness - that's the quickest way to make her stop trusting you and get defensive. What works better is asking specific questions and letting her answer without interruptions. "What exactly makes it worse for you? Is it the noise, having too many people around, speaking up in class?" That way you can tell if it's real anxiety or something simpler you can fix, like a quieter spot in the classroom or switching classes.
The thing is, at 14 many kids have legitimate difficulties with in-person school - for some it's genuinely anxiety or sensory sensitivity, for others it's socially complicated - and in other cases it's a mix of everything plus a dose of "I don't want to wake up early". All of that can be true at the same time. If she can do online fine, that already gives you a clue: it probably isn't pure laziness, because online also takes discipline.
Ask the school to consider a middle-ground plan - maybe two days in-person and three online, or a trial period - while you both look into whether there are mental health issues with a professional. And in the meantime, listen to her without judgment. When a kid feels like she's actually been heard, it's much easier to have difficult conversations later or set boundaries that she'll respect. If it really is laziness, she'll give you signals when she sees you're taking it seriously and there are actual steps being taken.