Felpham Community College sent a letter to parents at the school saying they were 'delighted' with the 'good' rating after the inspection they were given 24 hours notice for.
Part of the report said: "Since the last inspection, leaders have developed meticulous systems to track pupils’ progress at key stages 3 and 4. This ensures that any pupil at risk of falling behind is quickly spotted.
"A wide range of effective additional help is then provided to help pupils until their progress matches the strong rates of progress of other pupils in the school with similar starting points.
"You are now rightly introducing the same careful monitoring of students’ progress in the sixth form. However, your focus goes beyond ensuring that pupils leave with good examination results. Pupils’ personal development and well-being, including their mental health, have a high priority and are well provided for at your school."
Headteacher Mark Anstiss said: "We are delighted to have maintained our good status. In January 2016 we originally had a good Ofsted following a two-day full inspection which was a significant milestone.
"With funding pressures that schools are under you are always concerns with standards slipping but we want to stay as good and get better."
Mr Anstiss said the latest inspection was 'a bit of a surprise' but the called the results 'reassuring' for the school and parents.
The school is currently undergoing a £11m expansion which will allow them to take on more students and more staff in the coming years.