Ya snooze, ya lose… if they don’t send their work by your deadline, they don’t get to have it on the poster.
I like the header font you chose, and the triangle format.
But - keep in mind that this poster has a job to do. It needs to get viewers’ attention quickly. So make decisions based on the viewers. A professor of mine once said, “First, make it work. Then, make it pretty.”
Visual hierarchy means deciding which elements are more important than others. so their font sizes should be larger. I’d also look for another second font that is more interesting.
Consider changing that streaky designer triangle from color to black, or maybe a subtle dark, dark gradient. The color is drowning out and competing with the art pieces, and making them less important. Readability at the top would be better too.
The other thing I’d do is enlarge the art samples. If the poster is about an art show, the poster should show art samples as very important. It would be more interesting to art lovers, and the artists will love it.
Before you finalize it, print it out on whatever size paper it will be on, and hang it up. Then walk away, then walk toward it and test how easily you can read it and how far you are when you first notice it. The words “Art Show” should be as large as possible while still fitting in the design. And though I like the font, it may not be readable enough. You’ll know when you do this test.