Equality campaigners have voiced outrage that a boy lost his finger after reportedly fleeing bullies.
Raheem Bailey, 11, caught his finger climbing a fence to escape, and it could not be saved despite surgery.
His mother Shantal said her son, a pupil at Abertillery Learning Community in Blaenau Gwent, had been subjected to "racial and physical abuse".
Boxer Anthony Joshua and footballer Jadon Sancho are among those to send messages of support to Raheem.
An online fundraising campaign set up by Ms Bailey has already received more than £94,000 in donations.
Race Council Cymru chief executive Prof Uzo Iwobi said she worried people still did not believe black lives mattered.
All campuses at Abertillery Learning Community were shut on Monday on health and safety grounds and students used blended learning instead.
Blaenau Gwent council said: "Due to the serious nature and content of some of the social media activity over the past few days, a decision was made on Sunday evening on health and safety grounds to move all Abertillery Learning Community campuses to blended learning on Monday 23 May.
"The safety and well-being of all learners and staff remains paramount. We have continued to closely monitor and review the situation and can confirm that all campuses at Abertillery Learning Community will be open to learners as normal on Tuesday 24 May."
Ms Bailey said her son was "truly brave" but had been "in utter agony".
"The whole time [he was] telling me 'I'm sorry, I'm sorry Mummy. I just couldn't, I couldn't stay there, like why does no-one like me? Why did they pick on me?"
"These are things that my child, while being in pain, is constantly having to ask me, 'Mummy, why does no-one like me? Like why? Why did they pick on me?"'
Prof Iwobi said: "When will we know black children are going to come home alive and with their fingers and toes attached? I am in fear for all of our children that they could come back in a box."
"I'm absolutely disappointed, disgusted and outraged that this is the experience of black children in Wales.
"I feel nothing has improved. Teachers and the educational leaders of schools need to be especially sharp to detect racism and racial harassment and they are not doing that.
'When will black lives matter?'
Education Minister Jeremy Myles said in a statement that he was "shocked and saddened" to hear of the incident.
He added the Welsh government condemned bullying and harassment in any form, including any form of racist bullying or harassment.
"Every one of our education settings in Wales should be an inclusive, engaging environment where everyone's well-being is considered and everyone feels accepted, safe and ready to learn," Mr Myles said in a statement.
"We expect allegations and incidents of bullying and racism to be fully investigated by schools, with appropriate action taken to address the matter and prevent further instances from happening."
Prof. Iwobi, whose own children have faced racism in school while growing up, said: "This boy could have lost his life trying to get away."
She added: "The question I want to ask every teacher and local educational authority is 'When will black lives matter?'
"This boy will have four fingers for the rest of his life. His life is now marked by a racist incident and all he did was go to school."
Ms Bailey has said her son was "attacked" on Tuesday by a group of children who kicked him when he was on the floor.
She explained how he caught his finger in the fence trying to escape and after surgery it was decided that it had to be amputated.
"I am grateful and happy he's alive," she said.
"He actually said he was worried and thought that he could have died, he just couldn't get away no matter what he did."
Hadassah Radway, an education consultant for Black History Wales, told BBC Radio Wales Breakfast she cannot imagine what Raheem is going through.
She said: "This is going to be haunting him for the rest of his life and also his younger siblings. What must they be thinking?
"They might be thinking, this is going to be my lot, is going to be my situation in going into a school system that should be looking after them."
Wales' children's commissioner Rocio Cifuentes said it was "heart-breaking" what had happened to Raheem, and she will be looking into the case "with urgency".
Heartbreaking to hear what happened to Raheem 💔I will be looking into this case in more detail and more broadly what can be done to ensure all schools are effectively responding to racist bullying in Wales - this was already on my agenda but this case highlights the urgency 😔 https://t.co/ZfBMzZFKs7— Rocio Cifuentes (@rocdaboss76) May 22, 2022
Ms Bailey said Anthony Joshua, Jadon Sancho and footballer Ashley Williams - a former Wales captain - have all sent messages of support for Raheem through Instagram.
Ms Bailey said Raheem also received messages from football manager Chris Hughton, pundit Gary Neville and Olympic BMX biker Kye Whyte.
"Here's so many people just in different places that have been so generous, and I did not expect what has happened so I am truly, truly grateful for it," said Ms Bailey.
She said the messages he has received mean so much, as people tell him "how strong he is and that this does not define him".
Speaking about the wider issue of bullying and racism in schools, Ms Bailey added: "Why should I send my child into school to be a punching bag?"
Growing tearful, she said: "It is difficult, as a mum, having to tell your child that people might not like you because of your skin - not because you're mean, not because you're horrible, but just because of the skin he was born into."
Gwent Police said said the alleged attack was reported to it at about 13:00 BST on Wednesday, 18 May,
It said it was working with the school as part of its inquiries.
Abertillery Learning Community said it was working with the police and the council.
A Welsh government spokesperson said: "We condemn bullying and racial harassment in any form and expect allegations and incidents of bullying and racism to be fully investigated by schools, with appropriate action taken to address the matter and prevent further instances from happening."