The ETA we will provide is valid for a maximum stay of 3 months in Australia. 
You may study up to 3 months in Australia with the ETA, BUT if you wish to stay in Australia for a period longer than 3 months then you will have to exit Australia and  fly back to your country, get an appointment with the Australian embassy and stamp the student visa in your passport. 

We recommend that if you are staying for more than 3 months in Australia to study then you need to apply for a student visa at your nearest Australian Embassy.