A clash between the leaders of the ACLU and city schools highlighted the tension building in Baltimore as jurors on Tuesday deliberated the fate of the first police officer to face trial in the April death of Freddie Gray.

Officer William Porter, 26, could face 25 years in prison if convicted of manslaughter, assault, misconduct and reckless endangerment. Porter is the first of six officers to face trial in the death of Gray, an African-American who died one week after being severely injured while in police custody. The tragedy touched off a series of sometimes violent protests, and Baltimore officials are trying to avoid a repeat of those angry days.

With that in mind, city schools CEO Gregory Thornton issued a memo to parents this week saying schools will use the "situation" to mentor students in "appropriate ways to express dissent" without violence.

"We need to make it clear that student walkouts, vandalism, civil disorder, and any form of violence are not acceptable under any circumstances and that students who participate in such behaviors will face consequences," Thornton warned.

The letter drew an angry response Susan Goering, executive director of the ACLU, who said student walkouts are a form of acceptable, non-violent civil disobedience. Goering said Thornton's letter "assumes students would engage in violent acts, assumes that students only want to express their emotions, not rational views about the conduct of police and lack of accountability, and it misses an opportunity to affirmatively engage students who want to be politically engaged on these issues."

The city has taken steps aimed at keeping the peace no matter what the verdict. The police department canceled leave for all officers this week while Mayor Stephanie Rawlings-Blake opened an emergency operations center and pleaded for calm.

"Whether you agree or whether you disagree with the jury's ultimate verdict, our reaction has to be one of respect in Baltimore's neighborhoods," she said.

A few hours after beginning deliberations Monday afternoon, the jury sent its first questions to the judge, asking for transcripts of police dispatcher tapes and a police interview with Porter.

A court spokeswoman said the judge denied the request, since no transcripts were introduced as evidence, The Associated Press reported. But jurors can review the audio and video.

Jurors — four black women, three black men, three white women and two white men — also asked for definitions of terms from jury instructions. The terms are "evil motive," ''bad faith" and "not honestly." They relate to jury instructions for the misconduct in office charge Porter faces. As for the definitions, the judge said he could not expand on his jury instructions.

Porter, who is black, did not participate in the April 12 arrest of Gray, but he is accused of breaching police department protocols by failing to buckle the shackled Gray into the seat belt of a police van and then failing to immediately seek medical assistance when Gray requested it.

Gray, 25, suffered a severe spinal injury while being transported after his arrest. He died a week later.

Porter testified last week that he was in the van for most of the 45-minute ride between the site of Gray's arrest and the Western District police station. Porter said Gray did not appear injured and kicked and yelled for most of the journey. Porter added that Gray didn't ask for medical assistance but agreed when Porter asked him if he wanted to go to a hospital.

Porter said he asked about the hospital because he knew the jail would not accept Gray if he claimed to be injured. The driver, Caesar Goodson, declined to go to the hospital, instead stopping to pick up another prisoner, Porter said. When the van finally arrived at the police station, Gray was unresponsive.

Gray died a week later. His highly emotional funeral on April 27 drew thousands of mourners and protesters. The demonstrations turned violent, and dozens of people were injured while stores were looted and burned.

Rawlings-Blake fired Police Commissioner Anthony Batts in July, citing a spike in violent crime after Gray's death.