The good news is that these students are learning in a safe environment how to plan. Many people learn this skill on-the-job and often badly. We all have times when we fail to plan...fail to understand what they key activities are...fail to recognize who should be involved...fail to articulate the deliverables...fail to create a complete budget...fail...to...well, you get the idea. Failing sometimes has dire consequences. We may need to backtrack, pull in more resources, spend money that we hadn't allocated, ask for an extension, etc. Failing also means that we're stressed, as is the organization. Failing is never pretty.
Over the years, I have been surprised by the projects that have failed to plan. I once encountered a project team that was too busy to plan, yet couldn't get their project started because there wasn't agreement on what they needed to do. I've also encountered projects that were grant funded and were the details provided to the grant - the plan - were part fictitious. Success in those situations is harder...do-able...but harder.
If you have never planned a large project, do not despair. I know that you have actually informally planned things that have been successful. Now you need to learn a more formal, rigorous process for planning. You can do that through workshops, college courses, or even becoming a certified project planner through the Project Management Institute. You could also do reading on the subject. However, reading isn't enough because you will tend to ignore that parts of project planning that you don't like (or that you don't realize are necessary). Some sort of class will ensure that you are exposed to all of the elements of project management.
In the class that I teach, we use a variety of readings on project planning, creating marketing plans and creating assessment plans. Below are two of the textbooks/workbooks that I use. One is on marketing and the other is on assessment. Yes...those are two activities that all libraries should plan! If you don't want get these books...look for others. They're out there and they will help you be successful.