I have the same situation at my travel school. Here's what I do:

1) Make a Unit featuring a topic.
    EX: Restaurant
2) Think of vocabulary and set phrases that would fit the topic.
    EX:  "What would you like to drink?"
           "I would like..."
3) Make a lesson out of each phrase chunk and make a game or activity to go with it.
4) Optional: Tweak each lesson with harder vocabulary and various phrases for Grade 3 and easier for Grade 1.

My restaurant unit went like this:
  Lesson 1 - Vocabulary: Taste and Texture
                   Ss learn vocab related to taste and texture.
                   Activity - blind taste test: Ss taste a food and describe it using the new vocab

  Lesson 2 - Make a menu
                   Ss learn what goes on a menu
                   Activity - Make a menu: Ss design a restaurant and menu using taste and texture vocab words.

  Lesson 3 - "What would you like to drink / eat?" // "I would like..." // "Here you go."
                   Ss learn food and drink vocab and use it with target phrases.
                    Activity - Memory Game: Ss have flashcards of target vocab in pairs. The first S asks "What would you like to eat or drink?" The second S answers, "I would like..." The first S tried to guess where the matching flashcard is. (Like the matching card game Memory."

  Lesson 4 - "Excuse me." // "What can I get for you?" // "Can I get a..."
                  Ss learn dishes and silverware vocab and use it with target phrases.
                 Activity - Game: Ss are split into teams. One S is the waiter. The other Ss are customers and have a vocabulary flashcard. When the customers say, "Excuse me," the waiter asks, "What can I get for you?" The customer says, "Can I get a (vocab word)." The waiter runs to the front of the room and chooses the correct item and gives it to the proper customer. Fastest team to complete all the vocab words, wins.

  Lesson 5 - Put it all together.
                  Ss use all the menu, vocab and target phrases they learned to write and perform a skit.
                  Activity - Skit: I model an ad lib skit with one of the higher level students. The Ss must use one part of each lesson in their skit.

Hope this helps!

There are free textbooks online. Even if you only use them for 10 minutes, they're so helpful in providing structure.

I love damocha's suggestions.  Middle School students do like themes.  Food lessons are great and there is such much you can do with food.  I've done Food: Taste and Texture and a pizza lesson where they learn about the three different types of American pizza: New York Style, Chicago Pizza, and Detroit Pizza.  I had them practicing ordering pizza and side dishes.  I've also done a Supermarket lesson where they are asking where to find things in the supermarket and completing a roleplay where they are making a shopping list of food containers.  I did an Ice Cream lesson where they learn about different types of Ice Cream: Banana Split, Float, Sundae, soft ice cream, regular ice cream, etc.  They practiced a role play where they had to order ice cream and say whether they wanted their ice cream in a cup or a cone and whether or not they wanted sprinkles.    I've done a restaurant lesson where they practice ordering at a restaurant and learn about different types of restaurant.  Food is quite endless with what you can do with it. 

I've done Movie lessons where they learn about different types of genres, movie careers, different ways to describe a movie, talk about their movie preferences with a partner, etc.  You can do lessons on giving directions, gestures around the world, cell phone text messages language, British vocabulary vs. American vocabulary and spelling, animals and their habitats, Theme Parks and describing the different rides, health and sickness and describing one's symptoms and giving suggestions to make one feel better, super heroes and super powers, etc. 

Clothing is also something you can do a lot with.  You can talk about different materials, patterns, styles....You can get them to do all sorts of roleplays at a clothing shop.  Hair Styles around the world is also a good one.  Facial and physical body descriptions is also useful.   You can find material for all of these topics on waygook.  Another piece of advice that comes to mind is that in general it is better to make things a little hard rather than a little easy.  If a lesson is too easy then your students will lose interest in it.  Candy can also be really useful in getting students to practice role plays, etc.  Good luck with everything!