In what was United's third to last game of the regular season, the young side lost 7-2 to the Mariners following a turbulent ten-minute spell in the first half where the side were put to the sword with the weather conditions also playing a massive impact on proceedings.
Professional Phase Development Coach Steve Thornber said: "Looking at the two sides, they were all second year scholars, so were bigger, older and stronger than us. Four of the team were 15 or 16-years-old, which can make a massive difference.
"We were against the wind in the first half and Grimsby adapted better to dealing with the conditions that we did. We looked a lot better in the second half and made two changes at half-time and played more in the Grimsby half but they defended quite well.
"Overall, we were a young side coming up against a very strong, unbeaten Grimsby team. It was a disappointing first-half, but we got a reaction from the lads in the second half and it's a good experience and a good learning curve for them."
United went behind after seven minutes as the visitors came forward. Jake Balme made a good initial save from a shot coming in, but the ball dropped to the Mariners' centre-forward who was there to slot home the rebound.
Despite the final result, the Iron did draw back level, with that goal coming on 16 minutes. Trialist B, who has netted a couple of goals for the side in the past, slotted into an unguarded net in similar fashion to the Grimsby opener after Harry Lewis saw his effort saved by the goalkeeper.
From there, the game got away from the home side with the Mariners netting five goals in a 10 minute spell.
After 25 minutes it was 2-1 following a corner which was flicked on at the near post, with Grimsby doubling that lead within a minute following a swift counter-attack as Scunthorpe looked for an immediate response.
On 28 minutes, the score was extended to 4-1, before four minutes later it was 5-1 when a shot - which appeared to be heading wide - was saved by Balme, with the rebound again put in.
Ten minutes before half-time, it was 6-1 when the United backline didn't get the ball clear with the visitors capitalising to extend the scoreline further.
United made two alterations at the interval with one of those, Joey Dawson, having the first attempt of the second half when he drew the away shot stopper into a very good save.
On 58 minutes, the Iron pulled a goal back to make it 6-2 when Dawson threaded in a pass to Lewis, who sent a low shot across goal and into the far corner of the net.
The home side had a couple of further chances in the half, along with a couple of half chances, but were unable to reduce the deficit further.
The visiting side finished the scoring two minutes from time as the side broke forward on the counter-attack, with Oliver Kemp hauling down his man inside the box for a penalty. The spot kick was dispatched beyond substitute goalkeeper Ben Crosher for a seventh.
United are next in action a week on Saturday when they host Huddersfield Town.
IRON: Balme (Crosher, 65), Trialist A (Franklin, 75), Kemp, Young, Lobley, Cribb (Trialist C, 45), Gallimore, Wilson, Lewis, Trialist B (Dawson, 45), Poulter (Strouther, 85).