The teenager has been named locally as Will Morgan, who is believed to be a former pupil at the Gartree Community School in Tattershall.
Police have not yet confirmed his identity and say they want to speak to family members before making any announcement.
Messages began to appear on the Horncastle News website last night and today.
Beverley Allen Ashley-Rogers said: “Awful...so so sad xx thoughts are with family and friends. RIP Will”
Wendy Baxter said: “So sad. RIP Will. My thoughts are with the family & friends.”
Carol Eley added: “Thoughts to the family.”
Justine Terri Wright said: “Oh no :( thoughts are with the family.”
Paul Freeman simply commented: “Bad times.”
A Facebook page called ‘RIP Will Morgan’ was set up earlier today by friends.
Police say the collision on the B1191 near Thornton involved a motorcycle and a tractor.
They add that the driver of the tractor suffered shock but was uninjured.
Any motorists who were using the road at the time of the collision, who have not spoken to officers at the scene, are asked to call 101, quoting incident number 229 of May 4.