The most obvious answer to your question is a director has the right to manipulate a text. Unfortunately, this happens a lot and angers many readers. That being said, your question could be answered differently by any one person given interpretation of why the director chose to end the movie is subjective. Here is my reasoning.
At the end of the Speak text, Melinda is speaking to Mr. Anderson.
The last bell rings. Mr. Freeman comes to my table.
Mr. Freeman: "TImes up, Melinda. Are you ready?"
I hand over the picture. He takes it in his hands. and studies it. I sniff again and wipe my eyes on my arm. The bruises are vivid, but they will fade.
Mr. Freeman: "No crying in my studio. It ruins the supplies. Salt, you know, saline. Etches like acid." He sits on the stool next to me and hands back the tree. "You got an A+. You worked hard at this." He hands me the box of tissues. "You've been through a lot, haven't you?"
The tears dissolve the last block of ice in my throat. I feel the frozen stillness melt down through the inside of me, dripping shards of ice that vanish in a puddle of sunlight on the stained floor. Words float up.
Me: "Let me tell you about it."
At the end of the movie, Melinda is talking to her mother. Perhaps the reason behind the difference is that the director of the movie was trying to encourage open communication between mothers and daughters, instead of the realtionship between a teacher and a student. While the text obviously makes more sense, Melinda was much closer to Mr. Anderson, it could be construed in today's society to be inappropriate.
In the end, the importance lies in the fact that Melinda talked to somebody. After all that she had been through, the ostrization, the bullying, and the scandles, Melinda was, and is, a survivor. So, how the novel and movie's ending differ is a wonderful observation, it is much more important to realize that she opened up in the end. The real message behind the text in the first place.