Christina Edkins, 16, sent a tweet about the man, The Sun reported.
"This man is worrying me on the bus, keeps getting up and walking up and down stairs, and sitting in different place," she said.
Edkins was stabbed Thursday morning, minutes after she got on the bus and sat down on the upper level. Five hours later, a 22-year-old suspect was arrested nearby.
The suspect, identified as Phillip Simelane, was hospitalized Friday under the Mental Health Act, the Daily Mirror reported.
A schoolboy witnessed the attack, police said. A woman, who administered CPR to Christina, said on Facebook she encountered the attacker as he left the bus.
The stabbing was also captured by security cameras, with footage showing a man wearing a hooded sweatshirt getting out of his seat as the bus approached a stop, lunging at Christina and then walking off the bus.
"This was not a frenzied attack. The man walked up behind her and stabbed her," Police Detective Superintendent Richard Baker said. "There was no conversation. It was all over in seconds."
Christina , her brother and sister and parents lived in central Birmingham. She was a student at a high school 10 miles away where her parents thought she would get a better education.