I know someone whose oldest son got in but the younger brother didn't because he hadn't been baptised before 6 months,
Yes they brought that rule in at a lot of RC schools when Eastern Europeans immigration took off. It is an English/Irish (not sure about Wales and Scotland) custom to baptise as early as possible. My brother was born 30th November and baptised before Xmas.
In many Eastern European countries it is tradition to baptise at 1 year.
Also many Eastern European immigrants want their children baptised 'at home' where grand parents / friends / family can attend. It's not easy to get a passport, arrange flights, arrange church / baptism etc in 6 months.
Not that RC schools are racist. No not at all.