Raiders climbed over the roof of Morton Park Primary School in Carlisle and went on a rampage in the school garden.
Five frogs and three fish were found lying dead by the pond on Monday.
Head teacher Val Bairstow said the vandals had killed the school's entire collection of frogs and fish, leaving children "very distressed".
A spokesman for Cumbria Police said the force had been alerted to the break-in shortly after 0700 BST on Monday.
'Particularly horrific'
Staff at the school had noticed the damage but it was not until later, when the children went to look at the mess in the garden, that they spotted the remains of their pets.
The police spokesman said: "Damage was caused over the weekend when a group of offenders climbed over the school roof and into the yard.
"They caused about £60 damage to roof tiles, destroyed the garden and killed fish and frogs in the pond."
A spokeswoman for the RSPCA said: "This incident is particularly horrific.
"Not only did those responsible cause deliberate suffering and mutilation, they also caused the maximum amount of distress possible by leaving the fish and frogs to be discovered by the children, who were obviously then very distressed and traumatised."
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