VS Code has a C# debugger on Linux. There are certainly limitations to the open source C# support in VS Code, but for students it should be easily sufficient.

If you want a language where not only the core and main libraries are open source, but also all the tools then C# is probably not the right choice. If free (but not open) tools for most use cases and paid tools for larger companies or higher-level demands are okay, then C# fits.