How would they know it's from an iPhone as opposed to an older digital camera? 

If you don't resize it too far, the image displayed on a screen will be fine. I doubt they'll zoom in to check out every little detail so as long as you see what you want to show, move on to the next thing. Most screens are not Retina or high-resolution and the larger they  are, the less dense pixels are. Which works to your advantage if you work on a smaller but higher PPI screen. 

That hits the nail on the head. I take construction site photos with my phone half the time because they are better than the office cameras. As long as you don't pixelate it or make it look bad, you are fine.

I use my iPhone to document site conditions daily. I don't need to carry around 2 cameras. As for portfolio, as long as they are crisp, no one will care and know they came from a phone.
Shot, now that I think about it, my grad portfolio had scanned photos from actual film negatives. Damn.

Most smartphones are as good as standalone point and shoot cameras. Which I think is more than adequate for a portfolio of your models. Even if you are using an iphone, just make sure you compose your photos well. Have enough light and with a clean background. You can also take it into photoshop to modify it even more. I usually trim out the background and save my model images as png files.