Microsoft project management software used to plan, manage, and communicate a project schedule and other information among workgroup members, project managers, and other stakeholders.
The main difference between Project Plan 1 and Project Plan 3 is that Plan 1 provides the core web-based project management capabilities, while Plan 3 includes more advanced project management features such as the Microsoft Project desktop application, resource management, baselines, critical path analysis, and portfolio/roadmap capabilities.
Project Plan 1 does not include the Microsoft Project desktop application. Plan 1 provides project management capabilities through the web and Microsoft Teams experience, making it suitable for basic project planning and task management.
Project Plan 3 includes the Project Desktop client as well as additional features such as resource management, baselines, critical path analysis, and advanced scheduling capabilities.
For a class of approximately 30 students, I would generally recommend assigning licenses to individual student accounts rather than having everyone use a single account. This provides each student with their own workspace and aligns with the user-based licensing model.
If the students only need the web experience, Plan 1 may be sufficient. However, if the coursework requires the Microsoft Project desktop application, Plan 3 would be required.
Ref: Microsoft 365 Planner: Compare Plans and Pricing
You may also want to check whether your institution already has access to Microsoft 365 Education licensing. If available, you could create student accounts within the school's tenant and assign the required Project licenses directly to those students.