You seem to forget the time period between conceive and give birth, let's count the 40 weeks into 0.8 year.
Emily is 11, so counting the conception time, mom's age at that time will be 33-(33/3+0.8)=21.2
Mom is a bit older than alyvia when she got pregnant, let's suggest the most logic time of "a bit" is 1 year older and alyvia's age is "Z", then it will be Z+(Z+1+0.8)=33 and Z=15.6, mom at that time will be 33-15.6=17.4
Mc's age will be 11.8>X<15.6 and mom's conception age at that time will be 18>Y<21.2 by including the 0.8 year of pregnancy period
These calculation make all of them a little down age. The X,Y,Z school clearly represent primary, middle and high school in reality, the average graduated student age of middle school is around 15 and that don't not match with the calculated age of mc since he will soon graduated from Y school and go to the Z school.