I think it's better to vote for a party which has no chance of winning than to spoil your vote. At the very least it communicates what kinds of policies you would like to see and what policies would win your vote in the future.
I constantly think about the 2015 general election and how UKIP got almost 4 million votes (the third highest number of votes amongst all the parties). I feel that this caused a shift within the Conservative party towards populist, Eurosceptic, and anti-environmental ideals because they realised by doing so they could win back those 4 million voters.
I would personally never spoil my ballot for this reason. I don't think it's especially valuable to communicate that you're not happy with anything without communicating what would make you happy.
I'm currently in a circular debate with myself as to whether to vote Labour or Green. The classic eternal debate of "splitting the left vote" which we must deal with since we use an archaeic First-Past-The-Post system which should not exist in any modern democracy. I don't even especially like the Greens but a vote for them may communicate that one of my biggest values is preserving the environment and tackling climate change. Perhaps this could encourage Labour to establish policies to address these things in order to win back Green votes.
This is the thing that horrifies me the most about this story. Adults, schools, and parents are setting an abominable example to these children.
I can only imagine the confusion and shame a child must experience when being told to hide their insulin pumps, their wheelchairs, their hearing aids, etc. And I'm frightened to think of the pupils who feel empowered to "other" their classmates because they are being "othered" by the adults. It's a clear example of how we teach children bigotry.
An experience from my childhood which still sticks with me to this day is from when attending an ultra-orthodox church. I was maybe 5 years old and tried to follow my dad into a restricted area and being stopped by the priest, being told "sorry, only boys are allowed back here".
As a child I was taught that adults are always right, and to listen to them. This may very well be my earliest memory of being taught sexism, which only got reinforced throughout my life due to trusting the adults at this church and through trusting my very religious right-wing father. Even as a kid I recognised that what I was witnessing was unfair, but I did not have the power, the understanding, nor the will to challenge this unfairness because the adults must know what they're doing... right?