And the investment is paying off: Of the district's kindergartners deemed "fully ready," 84 percent came from a city schools' pre-K program. The only students who performed higher were a group of 137 students who came from private nursery schools.
But Alonso and other leaders are concerned that after rapid growth, the number of students in pre-K programs has hit capacity, at about 5,000 students.
That's about 2,000 less than the number of kindergartners enrolled in city schools this school year.
District leaders said the city is looking for more sustainable ways to serve more students, but they are barely maintaining the $29 million-a-year program in 109 schools.
School systems receive no additional funds for pre-K and are required to support the programs from K-12 budgets.
For the past several years, the program has been supported primarily by grant funds. But this year, the district is taking a deep cut in federal grant funding and will support the program mainly through its operating budget.
"We will not be fully successful until we can do it for all kids and until we start interventions even earlier. In the ideal world, we would start with 3-year-olds," Alonso said. "I think the payoff for the city would be immense if we could do that."
On a recent day, Mary Ann Winterling's kindergarten students enthusiastically wrote the signs of spring: walk-ing, swim-ming, grow-ing.
Teacher Johanna Fleury said gerunds would have never been the lesson of the day when she started her career.
And she looks forward to the years to come when students consistently come to her kindergarten classes fully prepared.
"It has almost shifted my job in that I can jump right in," she said. "You can push them. It's almost drastic. I'm doing things now that eight years ago, I couldn't do."
Baltimore Sun reporter John Fritze contributed to this article.