Recently installed fencing around the outdoor play area at Barndale House School was damaged.
The incident at the Howling Lane site is believed to have happened on Thursday, January 30.
Headteacher Daley Barber-Allen said: "Our entire school community has been shocked and saddened by this and other deliberate acts of vandalism that have been happening within our locality recently.
“We recently paid over £10,000 for new fencing to make our outdoor play area safe for our children and young people to play in and within days it had been vandalised.
“Why anybody would want to target a school, especially a school for children and young people with special educational needs, is beyond belief.
“We are trying very hard to raise money to improve the facilities at our school so when something like this happens, it causes a lot of upset and frustration for all pupils, staff, parents and friends of the school."
However, the school has been heartened by the volume of support it has received on social media since the event.
‘We have been amazed by the support from our community following this incident,’ it posted on its Facebook page.
‘Thank you to everyone who has contacted us with information - all details have been forwarded to the police.
‘The contractors have been brilliant and were here at 9am (Friday) to make the fence safe for our pupils.’
A Northumbria Police spokesman said: “Shortly after 2.05pm on January 31, police received a report of damage to a property on Howling Lane, Alnwick.
“It was reported that fencing and a sign belonging to Barndale School had been damaged.
“inquiries are currently ongoing and extra patrols in the area are in place.”
Anyone with any information about the incident is asked to contact police via 101 and quote reference number 575 31/01/20.
Barndale House School is an all-age special school for children and young people aged between two and 19 years with a diversity of learning needs and different abilities.