If you have seen any of the videos of the hearings in the courtroom you will see how the courtroom is set up and how it is totally possible that the Judge did not see the writing on t-shirt. In every video I have seen of the proceedings, the defendant is seated at a table, and does not face the judge head-on.
This article talks about how they think he was able to get into court with the 'killer' t-shirt on:
How Ohio School Killer TJ Lane Snuck 'Killer' T-Shirt Past Authorities
ABC News
By CHRISTINA NG
March 20, 2013
"Chardon school shooter TJ Lane likely got up in the middle of the night before his sentencing to scrawl the word "killer" across a prison T-shirt, a brazen move that shocked his attorney, victims' families and even the judge, according to the Geauga County Sheriff's Office.
After the convicted killer arrived in court for his sentencing hearing on Tuesday, he unbuttoned his blue button down shirt to reveal the T-shirt. There were gasps in the courtroom."
snip
"Lt. John Hiscox, Jr. of the Geauga County Sheriff's Office said that inmates at the jail are allowed to have three t-shirts, underwear and socks in their room. They also have pliable rubber pens for writing letters and filling out commissary forms.
"What the sheriff thinks happened is in the middle of the night, prior to him going up to court, he had taken one of his T-shirts and wrote the word on it, put the T-shirt on under his jail uniform," Hiscox said."
More / with videos ...
And, an article with video of what was said in court after the sentencing portion:
Geauga County judge didn't notice T-shirt TJ Lane wore in court that had 'KILLER' handwritten on it
NewsNet5
"Following his life in prison without parole sentence for the murders of Daniel Parmertor, Demetrius Hewlin and Russell King Jr., prosecutors asked that two things be put on the record:
1. After Lane walked into the packed courtroom Tuesday, he calmly unbuttoned his blue dress shirt to reveal the T-shirt reading "KILLER," which the prosecutor noted was similar to the one he wore the day of the deadly shooting at Chardon High School.
2. Lane turned in his chair toward the victims' relatives and raised his middle finger at them as he spoke, "The hand that pulls the trigger that killed your sons now masturbates to the memory."
Geauga County Common Pleas Court Judge David Fuhry replied, "the first I cannot confirm, but the second I can confirm for the record.""
More...
The kid has serious problems, imo, and I wouldn't want to be anywhere near him at any time in the future.