The city Department of Education released its annual school-space tally on Friday, months earlier than usual and featuring some changes recommended by an advisory group that includes parent representatives and principals.

The most significant revision to the yearly estimate of how much space is available in each school building, known as the Blue Book, is that it will now add students who attend class in trailers outside of school buildings into the main buildings’ enrollment counts. That should highlight how crowded school buildings will become once students are moved from the trailers into the main buildings, which the city has called a priority.

“This is a definite improvement,” said Leonie Haimson, executive director of Class Size Matters, a group that has criticized the way the city calculates available school space.

Some of the city’s fiercest school battles, including those centered on school overcrowding and the placement of multiple schools in the same building, revolve around the Blue Book’s numbers. Its calculations will likely come under increased scrutiny as the mayor’s pre-kindergarten expansion and a new state law forcing the city to find space for charter schools add to the city’s school-space crunch.

Critics charge that the Blue Book can underestimate how much space schools need and overestimate how much they have available, making the addition of more students or schools to a building appear less disruptive than they really are. In light of those and other concerns, Mayor Bill de Blasio had promised to temporarily halt new school co-locations, focusing especially on ones involving charter schools.

When his administration announced in February that it would in fact let some new co-locations proceed, as a concession the education department said it would form an advisory group comprised of community education council leaders, advocates, and others to review the Blue Book and suggest changes. Later, City Hall and the education department formed another advisory group to consider school space issues in general.

The head of the School Construction Authority, which helps compile the Blue Book, said at a hearing this month that the guide would be released early to give officials more planning time and would be more “user-friendly” than before. But she added that more substantial changes to the space calculations would not arrive until later editions. At that hearing, City Councilman Mark Levine worried that the changes would reveal more overcrowding and a need for more construction funding than was allotted in the latest budget.

Haimson, the class size advocate, said even with the improvements the Blue Book still contains “many, many flaws.” For example, she said it sets the class-size targets too high and does not allocate schools enough rooms for art and special education, leaving some students with disabilities to receive services in hallways and closets.