Apologies if you know all about this, I couldn't find anything much on the forum myself but I'm a fairly new Teensy and Windows user, albeit with lots of Mac and Arduino experience.

I was having issues getting the serial monitor to work (I know about selecting the port after the code runs) and tracked it down to an issue evident in Device Manager. It seemed that there were two devices trying to use the same port, the Teensy and something called 'Intel Active Management Technology'.

To solve this, within Device Manager you can get the properties of the Teensy (showing as 'USB Serial Device') go to 'Port Settings' and 'Advanced' where you can change the COM port number to something not in use. Once I had done this my serial monitor was printing as it should (sort of... its slow compared to a normal Arduino?)