[@Card Captor] Orion is just a title bestowed upon a student in each house who has shown that they perform "better" than the rest. It is more or less a subjective honour given to a student by the Headmaster and the School Officials. So a new student could not be an Orion, though a second year could. The Orion title is awarded to the student at the end of the year and carries over to the next year. This is why a first year cannot be an Orion, though if they earned the Orion title at the end of the year then they will be an Orion for the entire next year. However, the student for each house who wields the Orion title can be change every year. But it is also possible for a student to be an Orion for 3 years straight. But of course when an Orion graduates there is a new Orion to take their place so. 

Starborn is the highest ranking, Orion is simply a title. Though their are of course different levels in terms of skill and control amongst Starborns. 

To answer your second question, new students are informed in advance of which House they are to be placed in before they arrive. There is no magic sorting hat. lol 

Edit: To add on to that. A student can go through their entire 4 years at Star Ocean Academy as a Maeby, having never developed an ability while there. About 70% of the students at Star Ocean Academy are actually Maebies. If they manifest an ability they then become a Pre-Star, new and/or inexperienced or lacking control and skill with their ability.