Only half of Dubai students attend a private school rated at least 'good' by the government's education authority.
The 2012-13 inspection results released on Monday reveal that only 12 private schools, accounting for 20,435 students, in the emirate are in the 'outstanding' category, which is one more than last year.
A further 51 schools, with 87,663 students, are considered to be 'good', which is the Knowledge and Human Development Authority's lowest desired standard.
The largest percentage of schools (67 with 99,185) were rated 'acceptable'.
The final 13, accounting for 14,049 students, were unsatisfactory.
KHDA chief Jameela Al Muhaini said improvement had stagnated and there remained concerns, particularly in relation to school leadership, addressing special needs students and the teaching of Arabic.
The number of unsatisfactory schools remained the same as the previous year but the number of students surged 40 percent due to many of the five schools that fell from acceptable being large.
The proportion of students in the top schools has improved 2 percentage points from 2011-12, and 22 percent from the first year of inspections in 2008-09.
Schools using the UK curriculum again performed the best, with a record nine rated as outstanding. Three were deemed unsatisfactory.
Indian schools accounted for two outstanding schools, with a further seven rated good, 12 acceptable and two unsatisfactory.
The KHDA highlighted the lack of compliance among American schools, which continued to lag behind expectations.
"Many US schools are still not adhering to the standards of the US or the curriculum core standards, so they're not adhering to these international standards expected in the US," KHDA deputy director Fatma Belrahaif said.
Ten schools improved their rating compared to 2011-12. Four moved from unsatisfactory to acceptable, four from acceptable to good, and two from good to outstanding.
Meanwhile, eight schools achieved lower ratings.