I believe you will have to purchase a license in order to use the classic OPC DA, which offers no advantage over OPC UA. OPC UA and Modbus TCP drivers are included in the Codesys demo.
In my opinion Modbus TCP is the most straightforward to configure and use. However, if you intend to use analog values, keep in mind that Modbus registers only support 16-bit values. Factory I/O Modbus driver allows you to scale sensors’/actuators’ values as a workaround for this limitation. See more here:
https://docs.factoryio.com/manual/drivers/modbus-client/ https://docs.factoryio.com/manual/drivers/modbus-server/
OPC UA works better when using analog values and allows you to get a list of variables available in the softPLC. I would recommend you to try both and see which one would work best for your students.