Why did the ballot say "THIS IS A TAX INCREASE" - when in fact it is not a tax increase?
IndianFan answered the question but I'm going to respond as well to add some context.
Proposition A did not add a net property tax rate increase, but the rate will be higher than it would have been had there been no bond at all. Property taxes collected by school districts and other types of local governments are sequestered into two revenue streams. There's the maintenance and operations property tax levy (M&O rate), which funds employee salaries and the day-to-day operations of the local government, and the interest and sinking property tax levy (I&S rate), which is used by local governments to secure bonds for capital projects like new buildings, major renovations, etc.
The M&O rate tends to be set in stone for long periods of time. On the other hand, the I&S rate fluctuates depending on how much money the local government that pledged the I&S proceeds toward a bond needs to pay its obligation each year. Once a bond is issued, the I&S rate typically peaks very early in the life of the bond and then consistently goes down as the amount remaining on the obligation decreases and, ordinarily, the values of the properties against which the I&S rate is assessed go up. Right now, PN-GISD's I&S rate is on that downward slope because the amount the district owes toward the bond issue that passed in 2019 is falling.
The 2025 bond proposal isn't a big one. Truth be told, it's really a maintenance bond issue and, even as those things go, is a pretty small one. The effect of issuing the 2025 bond obligations will be to cause the district's declining I&S rate to either level off or decline at a slower rate--hence why the district is stressing that this bond proposal won't come with a net tax increase for PN-GISD homeowners. Nonetheless, the I&S rate will still be more than it would have been otherwise had the 2025 bond never passed and that triggers the state law requirement that the phrase "THIS IS A TAX INCREASE" be printed on the ballot.
There's a long explanation as to why state law requires that now. That's really a political subject for another day. I'll leave it off with these three bits of personal opinion: I think the law could be rewritten to more precisely state the actual circumstances in a situation like this one (though I doubt the Legislature will do that in the forseeable future); I still think the bond was the right thing for PN-GISD to do; and PN-GISD is very lucky to have pragmatic voters who understand the importance of investing in education and infrastructure, because the local election results across much of the area and the state last night were generally not very favorable to those types of investments.