Featherston School principal Gina Smith said the students were not told the filter was to be installed earlier this month, but the youngsters and staff had noticed the change in the taste and quality of the water.
"I did ask one what he thought of the water and he said it tasted like mud.
"I sent him out to have a drink after the filter was installed and he thought it was quite nice," she said.
"The staff have definitely noticed a change and are now drinking water from the tap.
"I personally used to bring my water from Martinborough and would only drink water from here if I really had to.
"We were unaware of any issues with our water.
"Our choice of bringing our own was based purely on taste."
Ms Smith said HRV and Water New Zealand, a non-profit organisation, had chosen schools at risk of water quality that was non-compliant and "we were one of the schools chosen and have had one of two filters we need put in place".
"We have all our main blocks and drinking fountains filtered and are now waiting for another to be fitted to filter the water into our technology unit," she said.
Installation of the water filter is free and HRV is also providing the first DIY filter change without charging, Mr Kara said, after which the school would be responsible for changing the filter at about $150 a year.
The goal is to have 60 schools around the country installed within a year in four stages, Mr Kara said, and Featherston Primary School was part of Stage One of the roll-out.
"The process and installation is different for each school.
"Firstly, the school fills out a pre-assessment form, which will give a good indication about whether installation is possible. And then an on the ground assessment is done.
"The filtration system is connected to the main water supply and it works by filtering out many heavy metals and chlorine, through to bacteria and water born bugs and nasties such as giardia."
HRV came up with the schools project to help improve the water quality of schools in areas that are most in need, to give something back to communities right around NZ, and to raise awareness of water quality issues in certain areas, Mr Kara said.
"It also means HRV get to demonstrate how our water filter works and what bugs, chemicals and other nasties it filters out."