The seder meal was a rabinnical invention that didn’t exist in Jesus’s time.
http://www.bib-arch.org/e-features/jesus-last-supper.asp
Oh, honey, have you been reading the wrong stuff! You DO remember the first Passover, right? When Moses led the children of Israel out of Egypt. Well ever since then God commanded that they commemorate that time with a feast that would last a week. Read Exodus 12, it's all about the Passover and the Festival of Unleavened Bread. Then in Exodus 34:18, God told the Israelites:
“Celebrate the Festival of Unleavened Bread. For seven days eat bread made without yeast, as I commanded you. Do this at the appointed time in the month of Aviv, for in that month you came out of Egypt.
In Numbers 9, God reminds Moses to remind the Jews to celebrate the Passover Feast while they are STILL in the desert. Deuteronomy 16, reiterates the same rules for the Feast. When Jesus and his disciples met in that upper room that last night, they WERE observing the Passover Feast, which a Seder is another word for that specific night. Matthew 26:17 speaks about that:
On the first day of the Festival of Unleavened Bread, the disciples came to Jesus and asked, “Where do you want us to make preparations for you to eat the Passover?”
Paul even spoke of this in I Corinthians 5:6-8
Your boasting is not good. Don’t you know that a little yeast leavens the whole batch of dough? Get rid of the old yeast, so that you may be a new unleavened batch—as you really are. For Christ, our Passover lamb, has been sacrificed. Therefore let us keep the Festival, not with the old bread leavened with malice and wickedness, but with the unleavened bread of sincerity and truth.
Here is a search of ALL the times the Passover was mentioned in the Bible http://www.biblegateway.com/keyword/?search=passover&version1=NIV&searchtype=all&limit=none&wholewordsonly=no
So, I'm not sure why you think Jesus didn't celebrate the Feast of the Passover which INCLUDED the Seder meal, but whoever says such things does not know Scripture. Jesus WAS an observant Jew as were his Apostles, so this was not an out of the ordinary thing he did with them, in fact I have no doubt they did the same the previous two years with him as well.