I was going to say many panniers are big enough to carry a preschooler, but since your daughter is in college I think her weight would throw off your balance, plus many racks have a 25kg limit.

But seriously...you may want to consider a Burley Travoy trailer or something similar (It's like a golf bag caddy/hand-cart) that can be wheeled into class.

If her portfolio is weather proof-ish, it can be bungied to the side of the back-rack with no pannier. But also it can be laid flat on top of the rear rack and bungied.