Three of the girls became so ill they ended up in intensive care.
Inspectors say they have already booked an appointment with the headteacher at Peder Skrivares school to find out more about how the alcohol poisoning happened.
"We are going down there because we think what has happened is very serious", the Head of Unit at the Schools Inspectorate Magnus Haglund told Swedish television.
The girls reportedly started drinking after school on Monday night, but it is unclear exactly what dares or tasks resulted in them getting so drunk.
Even though the initiation ceremony took place out of school hours, the Schools Inspectorate said that teachers could still be held responsible.
"We must ensure that the school is taking the right steps.We also need to know the real background to what happened," said Haglund.
All four girls are now understood to be recovering well.
The boundaries of schools' obligations to protect students also hit the headlines a year ago when eleven pupils at a boarding school in Lundsberg were burned with irons as part of an initiation ceremony.
The row went all the way to the Supreme Administrative Court, which ruled that school principals had a duty to prevent this kind of violence taking place in accommodation on school grounds. The school was eventually closed down.