It followed a similar success with Scotland’s biggest ferry operator, Caledonian MacBrayne, which said it had been persuaded to stop offering straws on its vessels by the children’s campaign.
And last December, pupils from the school on a field trip persuaded Ullapool to stop using plastic straws in all bars, restaurants and cafés, with help from Scottish Wildlife Trust and Ullapool Primary children.
Along the way, the school and its green champions have been praised at the Scottish Parliament by politicians including First Minister Nicola Sturgeon for their campaign, which uses the Twitter hashtag #NaeStrawAtAw.
The young campaigners are now applying pressure to turn pledges into action.
Recent days have seen the pupils make presentations to Glasgow council’s commercial catering division and to national dairy giant Müller – a producer of millions of plastic straws every year.
And there is more to this campaign team than just wall charts and paper models – they are currently busy rehearsing their presentations and fine-tuning their impressive knowledge.
Beth Ritchie, 11, is using a model gannet made of disposable drinks cartons and straws to get her point across. Indicating the bird, she says: “As you can see it has plastic straws in its stomach. The thing is, when it eats any plastic it thinks it’s full, so, sadly it ends up dying of starvation.”
She adds: “Glasgow city council are not recycling [straws]. So we calculated that 200 straws a day in our school are being thrown away – that’s 36,000 a year only at this school. So imagine the whole of Scotland, the whole of Europe, and the whole world.”
It is a point well made and stark in its simplicity, which is greeted with approval by the children’s teacher, Lisa Perrie.
Public awareness of marine pollution has reached a high-water mark since the BBC’s recent Blue Planet television series.
The children’s endeavours are now starting to gain traction – and headlines – where they didn’t before. But the scope of the children’s work goes beyond plastic straws.
P6 children are currently in the process of writing to US president Donald Trump about drilling for oil in the Arctic, and the impact on the seas and wider environment.
Classes also carry with them information cards that they pass on to people they meet to raise awareness of the plight of marine animals in captivity.
Mairin Hamilton, 11, explains how the campaign started. “The class of 2014 started the conservation campaign, The Sunnyside Ocean Defenders,” she says.
“Since then it has improved. All the other classes have taken on the role of conservation work. They all do their own different things. Every class helps the environment in their own way.”
Ms Perrie is rightly proud of the children’s achievements, and hails the all-round benefits of what is being done by the school. She said: “You can’t beat that impact for young children to know that adults at parliament, be it Holyrood or Westminster, are actually listening to them – that their opinion matters.
“They can go to these people with the knowledge and put their campaign across respectfully – it can have a huge impact.”
She adds: “Children are always drawn to nature, and drawn to animals. Hopefully what we give them are skills that they can then take out.
“And whether they’re next writing to their MP about housing or about a play park being built where they don’t want it, they have got these skills and then can hone them. The fact is that the planet actually needs them just now.”
The teacher adds: “Our motto is that we do not inherit the Earth from our ancestors, we borrow it from our children – and we haven’t looked after it that great.
“So they are beginning to speak up and they are beginning to put us in our place and tell us, ‘Pull your sleeves up adults, you need to get a move on with this and help us save the oceans and save the planet’.”
Others are quick to compliment the work of the Ocean Defenders.
SNP MSP Kate Forbes recently launched her Final Straw campaign to ask businesses and public bodies to ditch plastic straws.
She praises schools like Sunnyside for taking an active role in successfully lobbying groups to stop using plastic straws.
“Through their efforts, the Highland village of Ullapool became the first plastic-free village in the UK and so if pupils can achieve that in a matter of weeks, it begs the question as to why Theresa May needs 25 years to ditch single-use plastics.
“Since I launched the campaign, I have written to every council in Scotland to ask them to replace plastic straws with biodegradable alternatives.
“Almost half have responded, with Western Isles Council leading the way as the first council to go completely plastic straw free.
“I also wrote to the Scottish Parliament and I am delighted that they have agreed to go completely plastic straw free. That is leading the way, not just in Scotland, but across the world.”
She adds: “I believe that with my campaign and Sunnyside Primary School’s hard work, public pressure is really having an impact on public and private bodies.
“The key now is to take the campaign to the big food and drink chains who continue to give out millions of plastic straws, causing unbelievable harm to our seas and our wildlife across the world.”