The pro-gun meeting comes just days after the Uvalde mass school shooting in which 21 people, including 19 children, were killed by a teenage gunman.
“Before we nation-build the rest of the world, we should be building safe schools for our own children in our own nation,” he said in Friday’s speech.
US Congress have voted to send nearly $40bn (£31bn) in military aid to Ukraine as Russia’s invasion continues.
A total of $54bn has been sent to Ukraine since February.
Mr Trump told the conference: ”Congress should vote immediately to take back every penny of unused Covid relief money.
“Take it back from the states and use that money to quickly establish impenetrable security at every school all across our land.”
He also opposed gun control reforms, saying decent Americans must be permitted to use firearms to defend themselves against “evil”.
Mr Trump said he wanted to see a “top-to-bottom overhaul” of school safety, with new measures such as fortified single points of entry, metal detectors and at least one armed police officer stationed on every campus.
He also criticised the Democrats, claiming they had blocked these.
During his speech, he paid tribute to the Uvalde shooting victims, reading out each person’s name, marked by a bell toll.
The shooting, just 280 miles away from where the conference is being held, is the state’s worst school shooting.
Some called for it to be cancelled in light of recent events, while a few scheduled speakers and performers pulled out ahead of the three-day event.
Mr Trump made a remark about those who pulled out, saying: “Unlike some, I didn’t disappoint you by not showing up”.
Texas Governor Greg Abbott, who instead of speaking decided to address the crowd with a prerecorded video following the shooting, gun laws are ineffective at preventing mass shootings.
“There are thousands of laws on the books across the country that limit the owning or using of firearms - laws that have not stopped madmen from carrying out evil acts on innocent people and peaceful communities,” Mr Abbott said.
“In Uvalde, the gunman committed a felony under Texas law before he even pulled the trigger. It is a felony to possess a firearm on school premises. But that did not stop him.”
Mr Trump called in his speech for the US to “drastically change our approach to mental health”.
Meanwhile, Texas senator Ted Cruz said mass shootings were the result of absent fathers, a decline in people attending church, social media and video games.
He urged event-goers “not react to evil and tragedy by abandoning the Constitution or infringing on the rights of our law-abiding citizens”.
“If the left disarms America, those crimes would no longer be stopped,” Mr Cruz said.
“Many more single moms in subways would be assaulted or raped or murdered. Home invasions would turn much more deadly, as only the criminals would have guns. And far more children would be murdered.”
The NRA convention has seen protests taking place outside the venue, with signs such as “NRA kill kids” and “protect children not guns” displayed in opposition of the pro-gun lobbying group.