I think you need to make what's called a "style guide". On the style guide you show some sample assets (models, textures, and key concept pieces) which adhere perfectly to the style guidelines/art direction and technical specifications of the project.
It should cover as many different scenarios as you need it to in order to fully convey the art style of the project. More than just being a set of images it should also have explanations on why different design decisions were made so that the artists on the team can get a better understanding on what they should be doing as they work on their own assets.
Give a copy of the style guide and the sample assets to all the artists on the team and go over it with them.
Make some deadlines for different stages of each asset or group of assets, make the deadlines before you really need them. You should probably review each asset after modeling the high poly, modeling the low poly, UV unwrapping, and texturing are complete. Compare them to the style sheet at each stage of completion and make sure they are progressing in the right direction. Also check for any technical problems.
Communication is probably the key here. Get your artists to check stuff in with you more often so that corrections can be made before things need to be finished and less time is wasted. You don't necessarily have to have a sit down face to face meeting with each artist for each stage of each asset (which would probably be too hectic), find a way that you can give feedback on your own time in a organized way - see below:
If possible set up some sort of centralized place where everyone on the team can see the work being done by everyone else and offer up critiques, posting on Polycount is probably the perfect place for that since this is a student project and NDAs aren't in the picture.
Now take everything I've said with a grain of salt as I've never worked as a lead artist.