Class XI admission statistics for the current academic year reinforce how CBSE is weaning away high performers from ICSE, a trend that is telling on the city's top schools that are affiliated to the Council for Indian School Certificate Examinations (CISCE).

Most of the Class XI seats that CBSE schools keep aside for candidates from other institutes have been filled this year by students who did well in ICSE and chose not to make the natural transition to ISC.

Metro had highlighted on Wednesday how the exodus has forced the ICSE/ISC council to "rearrange" its plus-two science and mathematics syllabi to match that of CBSE.

At Birla High School for Boys in south Calcutta, students migrating from ICSE/ISC institutes have grabbed more than 55 out of the 60 seats in the science stream that outsiders could apply for. Delhi Public School Ruby Park has reported a 71 per cent intake of students passing out of ICSE institutes.

Ditto for Apeejay School Park Street, where 30 out of the 43 Class XI science seats that were open to candidates from outside have been taken by those from ICSE institutes.

More than the numbers, it is the quality of students switching to CBSE that is affecting the reputable ICSE and ISC schools. St James' School in central Calcutta has seen more than 10 per cent of its students who had appeared in ICSE 2016 migrate to CBSE for Class XI. It isn't a coincidence that all of them are high performers with scores in the 90s.

"This year, around 16 of our students left the school (from a batch of 130). This is double of last year's figure and dangerous. In 2014, only two or three of our boys had left...we are losing our best students," said Terence Ireland, principal of St. James'.

Pradip Bhattacharya, vice-principal of Delhi Public School Ruby Park, said: "In the last four years, we have seen a steady rise in the number of students from ICSE schools."

Many of the better students are opting out of ISC because of a perception that it would be easier for them to crack national-level competitive examinations for entry into engineering and medical colleges if they studied the CBSE plus-two course.

Gerry Arathoon, chief executive and secretary of the CISCE, admitted on Tuesday that the science and mathematics syllabi had had to be realigned to stem the exodus.

He hoped the changes, which take immediate effect, would convince ICSE students not to shift to CBSE institutes after their Class X examinations.

Heads of several CBSE and ISC schools in Calcutta said the trend that had started some years ago became more pronounced this admission season.

"We have been seeing demand for seats in Class XI science grow for a few years. Around 95 per cent of the plus-two seats that we keep for students from other institutes are being taken by students from ICSE schools," said Mukta Nain, principal of Birla High School for Boys.

Raja McGee, principal of Calcutta Boys' School, said on an average 10 per cent of his students were leaving after Class X to join a CBSE institute. He shared Arathoon's concern that most of the boys who had switched to CBSE were meritorious students. "I have noticed this trend over the past five years. The students who leave after ICSE are all in the 90 per cent-plus bracket and that is the reason it hurts."

According to McGee, there has been hardly any reverse migration from CBSE to ISC to balance the equation.

Sources in several CBSE schools in town said their reserved plus-two seats were attracting bright students from the best of ICSE/ISC institutes - Calcutta Boys, La Martiniere for Boys, St. James', Pratt Memorial, St. Xavier's, Don Bosco Park Circus, Julien Day and Assembly of God Church School.

The perception that CBSE students find it easier to crack entrance tests has been bolstered by the fact that the central board conducts some of the country's main competitive exams such as JEE-Main, JEE-Advance, AIPMT and NEET.

"Students coming to our institute from ICSE schools have admitted during their admission interviews that they preferred not to study ISC since the CBSE syllabus was more in tune with competitive exams," Nain of Birla High School said.

Subhendu Dutta Choudhury, the father of a student who switched to Birla High from St. James' this year, said the ISC council had been "trying" but was yet to mould itself to the requirements of national-level entrance tests that determine career paths.