It wasn't that Mabel misinterpreted his questioning as volunteering, so much as he was the only one who expressed interest in her artistic pursuits that she didn't feel any guilt for throwing the bait out there and seeing what she could catch. If he was that easily backed into agreeing, then she supposed he couldn't be that opposed to the arrangement.
From what she knew of the prefect, Ignatius wasn't the type to go back on his word, but she had been prepared in case he hadn't shown either, intending to paint that day either way despite her piling up homework she'd been neglecting. It was tough getting the art room to herself during the week with the various classes, so she fully intended to take advantage of it during the weekend- especially this time of day when the sun was providing perfect lighting through the windows for a good contrast of shadows.
"You're still dressed," she observed bluntly as way of greeting, peeking out from behind an easel in the corner of the classroom where she had been mixing thinner for the oil paints.