I always knew that even though Lisa was SEEMINGLY annoyed by Screech's unrelenting pursuit for her, she grew into appreciating that and ultimatally, started developing feelings for Screech! i.e.

-Remember when Screech and Lisa had a date to go to the movies? She actually enjoyed herself, and was all set to go to prom with Screech. He actually turned her down because she talked throughout the whole movie.

-In the 'Senior Prom' episode, she described his good qualities then took him to the Senior Prom, she even gave him a kiss!

-Remember in the 'Bayside Triangle' when Screech gave Lisa that necklace/locket with a picture of himself in it? That NEXT day, she was wearing it! At the end of the episode when screech said that he wouldn't forget her, (as she was holding his necklace which was still around her neck) she said that she wouldn't forget him! THE WAY that she said it it sounded really loving/romantic in a Soapy sort of way.