@shanm******
you have two options to get a 90-days Non-Imm-ED (education) visa:
*** apply for it in your home country before you fly to Thailand. You will need documentation from the school or university you plan to attend
*** apply for it on Immigration inside Thailand. This process is called “change of visa type” from a 60-days visa-exempt entry or from a 60-days Tourist Visa, to a 90-days Non-Imm-ED Visa. Also done with documentation from the school or the university
Under both options, the airline CAN ask you for a proof of onward travel out of Thailand within the period you will get stamped in, if you show them a one-way ticket at the Check-In counter