The man is alleged to have stabbed the Form Five girl in the neck with a knife and thinking that she was dead, attempted to kill himself. It is said the two were lovers.
The man allegedly waylaid the girl from Francistown Senior Secondary School near the Tati River bank and stabbed her.
Believing the girl was dead, he rushed home and stabbed himself in the neck in full view of members of the public.
He then grabbed a brick, tossed it up and cowered, apparently hoping it would land on his head and crush it but this did not happen.
The brick landed on his head but did not serious injuries.
It is said that for some time, the man had threatened to kill the girl. Parents and close friends of both the victim and assailant are said to know about the threats.
For the better part of Thursday, the man is said to have posted messages on social network telling his friends that he was going to kill himself.
The police said the man is aged between 20 and 30.
The police suspect the attack is a botched murder-suicide attempt.
Acting commander of the Central Police Station, Assistant Superintendent Edward Leposo could not divulge further details because both parties involved in the case are still recovering in hospital.
"Until we have met and questioned them, that is when we can disclose further details," he told The Monitor on Friday morning.
Leposo said that the suspect may face charges of unlawful wounding and attempting to commit suicide.
He advised professional counselling or help for partners to avert incidents such as this.
This is the second suicide-murder incident in Monarch in as many months.
The first involved a 26-year old man who stabbed his girlfriend with a knife and later hanged himself from the rafters of his house.
However, the woman, believed to be a college student survived.