Pretty sure this is Hazure Hantei kara Hajimatta Cheat Majutsushi Seikatsu.
One day, all the students in the second grade of junior high school were transferred to another world. The princess of another world summoned them to make up for the lack of strength. Students who are threatened and decide to follow the princess are given a magical check to check their abilities. While everyone was sorted according to the results, the results of Kento Kokubun's judgment were missing. I was banished because it was judged to be useless. However, there is actually a terrible ability hidden in Kento ... Kento, a second-year middle-aged student of life-size, aims to rescue his classmates with his fellow skeleton.
The entire third floor of a Japanese school building is transported to a forest in another world. The students within it were summoned by the third princess of Riesenburg, Camilla Riesenburg, who offers them the stark choice of temporarily serving in her army -- after which they'll be rewarded and sent home -- or being killed by her soldiers right there.
She also directs each of the students to place their hands on a crystal ball, in order to assess their magical skills and power level. Most of the students are indicated to possess abilities the princess is satisfied with, but the few who aren't -- including the protagonist, Kento Kokubun -- are deemed useless, and told to wander off into the woods and report to the barracks in the city beyond them.
Whilst walking alone through the woods, Kento is attacked by a band of goblins, which begin to eat him alive. Then out of nowhere, a group of skeletal warriors show up and start killing the goblins. Kento passes out, and when he wakes up, his injuries have healed, and the skeletons are stood in a circle around him. He asks what's going on, and they respond that they're under orders from their lord to protect him.
Looks like 18 chapters of this manga have been released in the original Japanese language, but the only chapter I've found currently translated into English is the first one.