Kenya’s Garissa University has reopened nine months after an al-Shabaab shooting rampage killed 142 students and six security personnel.

A staff meeting was held on Monday, with students expected back for lessons on 11 January.

Security has been reinforced, including the deployment of 25 police officers to a new barracks built within the campus. Garissa is the biggest town in the region, straddling Kenya’s long border with Somalia, which has been repeatedly targeted by militants.

“The reopening of the university marks an important milestone,” said principal Ahmed Osman Warfa. “We fought very hard to have this institution here and will not accept losing it.”

The six-hour siege of the Garissa college was the single deadliest attack by al-Shabaab outside Somalia. Four militants, including the son of a Kenyan government official, went from dorm to dorm separating Christian students from their Muslim counterparts and shooting them in the head at close range.

A survivor of the al-Shabaab attack on the university campus in Garissa, northern Kenya, in April 2015 is comforted by a colleague after arriving in Nairobi. Photograph: Tony Karumba/AFP/Getty Images

The killings left Kenya in a state of shock, and prompted criticism of the government that intensified with the news that one police chief took his family on a helicopter trip at about the same time elite forces who eventually ended the siege were stranded waiting for transport.

There have been security improvements in recent months, with hoteliers breathing a sigh of relief after Britain and France, two key source markets for tourists, lifted travel advisories against the coastal resort city of Mombasa.

However, al- Shabaab has continued to stage attacks on security officers near the Somali border.

A recent attack on a bus in Mandera in north-east Kenya saw Muslim passengers hailed as heroes after refusing to be separated from Christian passengers, forcing the attackers to retreat.

Kenyan police say they are worried that recent splits within al- Shabaab, in which a splinter group has declared loyalty to Islamic State against the wishes of the al-Qaida-aligned leadership, could provoke one of the factions to launch a major attack to prove its credentials.

The #garissauniversityreopens hashtag trended on Twitter in Kenya on Monday, with many hailing the reopening of the university as a psychological blow to the militants.

However, lingering security concerns are illustrated by the reluctance of students at the neighbouring Garissa teachers’ college to resume classes until significant security upgrades are in place.

Authorities say the 650 students who survived the 2 April attack would not be required to report back to the reopened Garissa University and would continue their studies in other campuses.