Formal:
* Je réside à Oxford, à l'école Summer Fields. (This is the most literal and formal translation.)
* Je suis élève à l'école Summer Fields à Oxford. (This emphasizes being a student at the school.)
Informal:
* J'habite à Oxford, à l'école Summer Fields. (This is a more casual way of saying you live at the school.)
* Je suis à l'école Summer Fields à Oxford. (This is a shorter and simpler way of saying you are at the school.)
You could also use "Je suis interne à l'école Summer Fields à Oxford" if you are a boarding student.
Remember to choose the option that best suits your context and the level of formality you are aiming for.